We always carry too much, prepare for too many eventualities. One bag could have been left behind. We are too afraid of unknowns to ignore them.
A Packing List
Valuables
Be sure to check the expiration dates on all your official documentation! Few things deliver the particular combination of embarassment, frustration, and inconvenience that come from a passport, driver's license, credit card, etc. having expired the day before you need it.
passport, visas, vaccination certificates, extra passport photos
Your passport is the single most important document when travelling outside your home country, so guard it well: almost all border agents (and often hotels and other establishments) will want to examine it. Many countries have additional, non-obvious entry requirements. Some will bar you from entry if your passport doesn't have at least six months' validity beyond your planned departure date (consequently, it's a good idea to renew your passport at least six months prior to its expiring, to avoid possible last-minute document
hassles). Some require vaccination certificates if you have previously visited areas where specific diseases are prevalent. Some insist that you show means of support (credit cards, or sufficient cash) while visiting, and/or return or onward air tickets. Many require visas, usually arranged before you leave home. One country (South Africa) has the rather bizarre requirement that your passport contain at least two completely blank pages, free of any stamps or other markings, and if you require a visa, the pages must face one another! So find out what you need before you go (you'll find some resources for this on the TraveLinks page), to avoid being refused entry, and sent home (at your own expense) on the next flight! Extra photos are useful for visas you might decide to obtain along the way.copies of important documents
If you should manage to lose your passport, an image (photo, scan, etc.) of the main page will make replacing it a lot easier. Similarly for prescription information, driving licenses, charge/ATM card account numbers, transportation & lodging information, traveller's cheque serial numbers, etc. Of course you'll carry the copies separate from the originals, won't you? It's also a good idea to record the special telephone numbers necessary to deal with the loss of (or other problems with) any charge/ATM cards you carry; this information is often on the backs of the cards. 
And though it's not suggested for government and other official documents, you can inexpensively protect many paper items by laminating them between two pieces of transparent plastic packing/book tape, as shown at left. Leaving an additional copy of all this information with a trusted friend back home can save you a lot of long distance calls in case of disaster (most toll-free numbers don't work very far from home).
Better still, consider using on-line (aka "cloud") storage for your backup information. One way to do this is by e-mailing document copies to yourself, thus effectively storing them on your mail server (Gmail, which offers free accounts with 15GB of storage, is particularly good for this). An even better solution is the use of a free Web-based file storage service (e.g., Amazon Drive, Dropbox, Google Drive, & Microsoft OneDrive), so your important information can be accessed from any Web browser. The widespread availability of Internet Cafés makes such Web-enabled backup solutions particularly practical (especially given that they effectively address the worst-case scenario, that of your losing everything). If you lack the capability to scan necessary documents, inquire at your local copy shop. It's useful to have copies in different formats (PDF, JPEG, Word, etc., as appropriate) to cover a variety of eventualities.
If you do manage to lose your passport, you'll likely have to obtain a replacement before being permitted to go home. This can generally be accomplished at any consular affairs office of your home country, often surprisingly quickly in emergency situations. They will, of course, want proof of your identity, which can be a problem in itself if your other identification was lost along with your passport (in cases of theft, getting the local police to provide a statement describing your situation can prove a big help). And having accessible copies of your documents as noted above will be useful, naturally.
U.S. citizens can avail themselves of the Smart Traveler Enrollment Program (STEP) when travelling (or living) abroad. This allows you to enter information about yourself so that the Department of State can better assist (or contact) you in emergency situations (including disasters, political unrest, etc.).
(international?) driver's license, health insurance information
In general, you'll want to leave at home most of those cards you carry around in your wallet; leave the wallet at home too. But there are exceptions. Should you need to drive, your regular driver's license (in conjunction with your passport) will almost certainly be sufficient. Some countries (particularly those with non-Roman alphabets) nominally require "International Driving Permits" (IDPs), which are simply translations of your regular license into ten languages (though, in practice, most officials have never heard of them). If you're unsure, though, check with a local authority (such as the rental agency you intend to patronize).
The "International Driving Licenses" one hears about (especially via the Internet) are essentially a big scam; before sending off your money for one, read this U.S. Federal Trade Commission report, which also has information about how to obtain a legitimate IDP.
Rental Car Considerations: 
A bit off the topic of travelling light, perhaps, but I thought it might be helpful to mention a few non-obvious aspects of automobile rentals. Foreign rental cars should, if at all possible, be booked in advance (from home); doing so after you arrive can be much more expensive. Your chosen guide book will often suggest lower-cost local alternatives to the major auto rental companies.
It is almost always cheaper to rent cars from locations well removed from airports (where the high fees paid to airport administrations are reflected in the rental rates). At this writing, for example, you can rent a compact Hertz sedan at the Toronto airport for a base rate of USD$66/day, or you can take a USD$3 (or less) subway ride from the airport to a downtown location where the identical car can be had for USD$38/day. And, if you so choose, returned directly to the airport at no additional charge.
When you rent a car in Canada or the U.S., it's a given that it will have an automatic transmission. This is not true in other parts of the world, where a traditional manual shifter is not only the norm, but usually much more economical than an automatic. Similarly, air conditioning and power windows/brakes are not always part of the standard package. If any of these is a concern, be sure to verify what you are renting.
Politely decline offers to pre-purchase gasoline (petrol): these schemes are often made to sound attractive, but have not been designed for your benefit. And ensure that the tank is full when you return the car, to avoid being overcharged for the fuel necessary to do so. For that matter, check that it's full when you initially receive the car as well (perhaps at the same time that you inspect the car for pre-existing damage, so as not to be blamed for it later).
Don't assume that liability, theft, and collision coverages on rental vehicles work the same way as at home; your own policy is unlikely to be valid in foreign countries. Many credit (not debit) cards provide some kind of automatic coverage when used to rent the car, but this is also very country-specific (Ireland, Iceland, Israel, Italy, Jamaica, & Mexico are often excluded), and should be confirmed in advance prior to your trip: telephone the toll-free number on the back of the card. If you do need to use a credit card plan to cover an accident, it's important to call the card issuer immediately; they will almost certainly require rental company paperwork. Personal health and home insurance policies sometimes cover injuries and/or theft; again, find out before you need to.
Finally, appreciate that driving conditions and regulations may differ markedly from those to which you are accustomed, especially with regard to rights of way and driving after using alcohol. In the UAE, for example, autos must yield to camels, having any blood alcohol can get you stoned (and not in a pleasant way), and you are required to pay compensation (known as "dhiyya", usually around USD$55,000) for anyone who dies in a motor accident in which you are involved. Know the rules (foreign affairs offices often provide useful traffic safety and road condition reports) and conduct yourself accordingly.
If you have health insurance coverage for the areas in which you are travelling, bring along some official-looking document that attests to same.
travel tickets
(i.e., airline tickets, rail & bus passes, hotel reservations, etc.)
Whenever you purchase an airline ticket directly (i.e., not through a travel agent) using a credit card, ensure that you bring that card with you on your travels: you can otherwise be denied boarding on many airlines.
charge & ATM cards, cash, (traveller's cheques?)
For purchases while travelling, use charge cards whenever possible (in much of Europe & Asia, this includes even taxis & fast food outlets); you'll get a much better currency conversion rate — generally 1% over the commercial bank rate, though see my comments on added fees, below — than by any other (legal) means. You'll also postpone the final reckoning. Make sure that your cards have sufficiently high credit limits to cover what you'll need (remembering in particular that automobile rentals will invoke a large credit hold against your card, until you have returned the vehicle). It's also important to call your credit card company prior to travelling far from home, to let them know your plans; this will save you the inconvenience (and possible embarrassment) of having your card refused because their computer doesn't know you're in Bangkok.
Beware the C4 Scam: You're in a restaurant or shop, about to pay for your purchase with your credit card, and the merchant asks,"Would you like your transaction to be done with <your home currency, like dollars> or with <the local currency, like euros>?" You hadn't even realized that you had a choice, but now that you do, you're tempted to pay with your (familiar) home currency. Don't do it! This is an old — and widely practiced — racket that I call the C4 (Credit Card Currency Conversion) scam, and falling for it could cost you plenty. First, you will have no control over the exchange rate applied (and your credit card issuer can offer you a better rate than any small merchant, including currency exchange booths). Second, if you let the merchant handle the conversion, there's nothing to prevent the addition of "foreign transaction fees" (sometimes as high as 7%) to the bill. So always pay with the local currency.
On occasion, you may encounter a merchant who insists that you let him do the currency conversion. In such circumstances, there are only two wise choices: pay in cash, or walk away.
Note that a number of charge card issuers (as distinct from the networks, such as Visa and Mastercard, which make their money from the 1% mentioned above) levy extra fees (often several percent!) for foreign currency conversions. If your card comes from one of these (find out before you leave), change cards, and let them know why you did. Really. These currency conversion fees are nothing more than a "tax" added by the card issuers, simply because they can; fortunately, not all issuers treat their customers in this fashion, so exercise your prerogative as a consumer and patronize the companies that earn your business. These rates tend to change without (or, more correctly, with well-hidden) notice, so contact your provider before you leave, in order to avoid an unpleasant surprise.
To obtain local cash, use a bank ATM card (make sure yours is part of of one of the global networks, like Cirrus or Plus); both MasterCard and Visa have Web-based ATM-finding services. It's better not to use credit cards for this, as you'll incur a cash transaction fee and begin accumulating interest immediately. If your bank levies a charge for foreign ATM transactions, don't withdraw USD$20 at a time. Note that an American Express card also acts as a cheque guarantee card, allowing you to cash a personal cheque at any American Express office (making this the best non-ATM cash solution).It's a wise idea to withdraw funds during business hours; that way, someone from the bank will be able to assist you in the event of an ATM malfunction (such as eating your card). At night, nobody is likely to be around.
Many banks have relationships with specific foreign banks, and will waive the foreign ATM fee if you use their machines. Check with your bank to find out about such partnerships, which are often listed on the bank's Web site.
Be aware that 4-digit Personal Identification Numbers (PINs) are standard in most countries; if you have a 6-digit PIN, you might want to have it changed. Better yet is to check the current requirements in the countries you are visiting; in China, for example, 6-digit PINs are commonly required, although there are 4-digit-friendly machines in Beijing and Shanghai. Should you get stuck with a 4-digit PIN at a 6-digit-only ATM, a technique that often works is to simply add a pair of zeros preceding or following your number. If you have two bank accounts, consider carrying both a 6- and a 4-digit card.
Yet another solution allows you to bypass most of the charges that are sometimes attached to the use of ATMs. This is the so-called stored-value card, available from Visa and MasterCard, which functions much like a telephone card: you load it with money and can then collect that money from ATMs along your way, paying only the local dispenser fee.
When withdrawing money from ATMs, it is often useful to request slightly less than one of the "default" amounts. If you withdraw €200 from a European ATM, for example, you might receive two €100 notes, which you could subsequently find difficult to cash (many businesses — especially smaller ones — don't like to accept them for small purchases). But if you withdraw €180, you'll get some smaller denominations, and more time to find a taker for the large one.
Keep some cash handy for essentials, but any significant amount safely hidden (see "security pouch" below).
Of Particular Concern to Those Travelling From (and To) the U.S.: 
Most of the developed world has made (or is making) the transition to "smart cards", using the EMV standard, also called "chip-and-PIN" (as the card contains a microprocessor chip, and identity is confirmed using a PIN, rather than a signature) and "IC Credit". Many ATMs, shops, and restaurants can still cope with the older magnetic-stripe swipe-only cards — though their numbers are steadily declining — but an increasing proliferation of unmanned, automated kiosks (at train stations, gasoline pumps, parking garages, bicycle rental stands, tollbooths, etc.) will only accept the more advanced cards.
So, given that the U.S. banking industry has never really committed to chip-and-PIN technology (although some suppliers have begun making available a similar — but not fully compatible — style of card, termed chip-and-signature), American travellers are finding themselves increasingly at a disadvantage in this regard. Those visiting the U.S.A. with cards lacking magnetic stripes will, naturally, encounter this compatibility concern in reverse. There's no simple solution to the problem; consequently, given that ATMs are among the potential candidates for inaccessiblity, those affected might find it wise to maintain a larger than normal cash reserve.
In response to this situation, some providers are selling stored-value smart cards for Americans to take along on their travels. While this seems like a reasonable interim solution to the problem, I advise you to evaluate such offerings very carefully: those I have seen to date appear — by virtue of their various "surcharges" and shipping/handling costs — more like cash-skimming operations than anything else.
Traveller's cheques have largely been relegated to the museum of tourism history; if you insist on being one of the few remaining users, bring more than you think you'll need, in both large and small denominations (some currency exchange places charge by the monetary amount of the transaction, some by the number of cheques converted). Stick with "major labels" (e.g., American Express).
As a rule, banks (and American Express offices) will give you a better deal on currency conversion than the many late-night bureaux de change you will encounter ("no commission" signs merely indicate that their cut is built into the exchange rate; conversely, good exchange rates imply high commissions). And plan to keep such exchanges to a minimum. When leaving a country, for example (and not planning a return in the near future), consider using most of your remaining cash toward paying your hotel bill (putting the rest on your credit card), rather than later paying to convert that money to a different currency.
When travelling in developing countries, where charge cards (and traveller's cheques) are largely useless, carry cash, in small denominations. The U.S. dollar is not as widely accepted as it once was (having been displaced by the euro in many areas of the world), but a mixture of one-dollar and five-euro banknotes, plus some €1 coins, is still a good approximation to a universal currency. A couple of hundred bills will not fit comfortably in your security pouch, so you will have to secrete most of them in various places about your pack (don't fret excessively about this; the loss of your pack will prove more of a disaster than the loss of a few hundred dollars).
If you do elect to carry currency, be aware that many places will not accept U.S. banknotes containing creases, tears, or ink marks. Or dated earlier than 2006. Or denominations higher than USD$20 or USD$50. I've heard several stories of people sitting in their hotel rooms, ironing their bills in an effort to make them look more pristine, hoping that someone might be induced to accept them!
Finally, pay attention to your money. Understand all charges included in any transaction (ask to have bills itemized). Be aware of local tipping customs (in Japan, for example, tips are considered insults; in many other places, they have already been added to your bill); tipping practices can be somewhat fluid, so check with a credible travel forum like The Thorn Tree before you leave, or ask a trusted local when possible. Particularly in highly touristed areas, assume that you'll be shortchanged; always ask how much, do your own arithmetic, and don't let yourself be hurried. Be friendly, but vigilant.
Banks and shops are normally reticent to make change for non-customers, so if you need a supply of coins for parking meters, vending machines, or whatever, remember that local self-service laundries almost always have change machines. And they are usually open for extended hours as well.
2 personal cheques
You may not use these, but they take almost no space, help establish credibility, and just might prove invaluable. Bring more on longer trips, should you plan to use them to transfer money from home (as mentioned under "charge & ATM cards", above).
security pouch
I can't overemphasize the necessity of this; it may be the single most important item on this list. Street crime is rampant in most densely populated parts of the world, and the more easily that you can be identified as a tourist (pretty much a certainty in most foreign countries), the more likely you are to represent a target. So never, ever, put any of the items (except document copies) from this section of the list anywhere but in a security pouch, worn under your clothes (and not accessed in any public place).
These pouches come in a variety of styles (two are pictured at right). The very safest are those worn around the waist, though I have always preferred the type that loops around your belt and hangs down inside your pant leg, which are lighter in weight, a bit more convenient, and much more comfortable (they can also be safety-pinned to the inside waistband of a belt-less garment, though I would want to use two safety pins to feel secure). Around-the-neck styles seem attractive, but their strings are visible with most clothing, and often flimsy to boot. And I find the "shoulder holster" versions to be less comfortable than the waist/belt types. Whatever your choice, look for light weight, comfortable-against-the-skin fabric, and sufficient dimensions to hold what's necessary. Get one of these, and get used to wearing it; it's the cheapest insurance you'll find.
Depending on the construction of your particular pouch, where it rests on your body, your propensity to perspiration, and the temperature/humidity of your destination, you may find that the documents inside can get damp. Should this prove to be true in your case, use a plastic liner of some sort (a Ziploc® bag in more serious instances) to protect contents that could be damaged.
Need more incentive to use a security pouch? Read Terry Jones' illuminating Barcelona Scams, one of the worst European cities for pickpocketing attacks, though similar activity abounds at many heavily touristed areas worldwide (such as Naples, in the following informative video). It's not that such cities are inherently more evil than others: it's that — unless you live there — you will be easily identifiable as a tourist, thus more likely to attract the attention of those you would prefer to avoid.
Those who can't bring themselves to wear an under-the-clothing security pouch can consider an alternative solution. Many travel wear manufacturers — ScotteVest, Tilley, and ExOfficio are particularly noteworthy in this regard — produce articles of clothing (jackets, shirts, slips, etc.) that have secret pockets sewn into them. These are not always sufficiently large to hold all of the items you should be protecting (i.e., everything on this page), so bear this in mind when choosing a design. Further, because you need to protect yourself from both pickpockets and more overt thieves, and because hiding something only works if it remains hidden, you must avoid the temptation to access such convenient hiding places in public (an under-clothing pouch helps to discourage this).
Thoughts on Wallets: 
Once you've safely stashed your passport, visas, driver's license, vaccination certificates, health insurance info, travel tickets, charge/ATM cards, cash, and cheques in your security pouch, do you still need a wallet? It's really up to you. Whether or not to use a wallet is a personal decision in any case; many people choose never to carry them. A modest amount of cash — no more than you can afford to lose — for the day's casual purchases scarcely needs a customized receptacle (especially one that is a prime target for theft).
In my experience, most people who do carry wallets cram them with far too much stuff … items that they really don't need to be carrying around in their pockets every single place they go (you are unlikely to need your library card in Bratislava). Not dissimilar to overpacking suitcases, although the motivations may differ somewhat. But even "at home", carrying too many official documents is a very unwise idea: in these days of rampant identity theft, losing a wallet can be a seriously life-changing event (and not one that you will remember fondly). In any event, never carry anything that includes any sort of national identifier (such as a U.S. Social Security number) in a wallet.
Storing a wallet horizontally in your pocket (rather than the common vertical position), with an open side upward, will make it considerably more difficult for pickpockets to extract. Make their job even harder by wrapping a rubber band around it (to increase the friction against your pocket). Rear pants pockets and inside jacket pockets are the easiest to pick, by the way (though none is truly pick-proof).
If you're one of those inclined to store valuables in hotel security safes, realize that it's all too easy to forget such things when departing. Place one of your shoes (or something equally unforgettable) in the safe as well, rendering the assemblage less likely to be left behind in the fog of morning.
Finally, in these times of increased airport security, with official/officious folks checking our identification/tickets/receipts/whatever at every turn, some travellers (especially parents travelling with children) have found it helpful to wear a handy neck pouch to hold all of these, making them instantly available when necessary (and recalling days in kindergarten when you wore a name tag around your neck). These are not intended as substitutes for a proper security pouch — and should not be used as such — but they can reduce one's stress level when en route via air. When you arrive at your destination, immediately transfer the important items to your security pouch.