Packing List
Documents
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
Almost all countries will want to see your passport, but many 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
copies of important documents
If you should manage to lose your passport, a photocopy of the main page will make replacing it a lot easier. Similarly for prescriptions, 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; 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). 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.
Consider using on-line 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 7+GB of storage, is particularly good for this). A better solution is the use of such services as Google Docs, Box, SkyDrive, and humyo, all of which offer free/inexpensive Web-accessible file storage, so your important information can be accessed from any Web browser. The dramatic rise in the 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.
(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. In theory, 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; 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 to obtain a legitimate IDP.
travel tickets
(i.e., airline tickets, rail & bus passes, hotel reservations, etc.)
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.
Note that an astonishing 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 (two to four 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.
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 also AAA. It's sort of 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.
Of Particular Concern to Those Travelling From (and To) the U.S.: Most of the developed world is making a 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". Much of Europe, Brazil, Japan, and México have already converted, and the process is well underway in Canada, China, India, and most of Latin America. Most ATMs, shops, and restaurants can still cope with the older magnetic-stripe cards, but an increasing proliferation of unmanned, automated kiosks (at train stations, gasoline pumps, parking garages, bicycle rental stands, etc.) will only accept the more advanced cards.
So, given that the U.S. banking industry has no immediate plans to upgrade its technology, American travellers are finding themselves increasingly at a disadvantage in this regard. Those visiting the U.S.A. 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.
Traveller's cheques have pretty much 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 or Thomas Cook). As a rule, banks (and American Express offices) will give you a better deal 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). Keep some cash handy for essentials, but any significant amount safely hidden (see "security pouch" below).
If you're 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 bills, 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).
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 taken as insults; in many other places, they have already been added to your bill). 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.
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 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. These are slightly less safe, but also 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.
If you need a little more incentive to use a security pouch, you might read Terry Jones' illuminating Street Scams of Barcelona. Not to be viewed as an indictment of Barcelona, it's merely typical of the sort of activity that abounds at many heavily touristed areas around the world (some are much worse!). And to reiterate, it's not that places like Barcelona are any more evil than other major world cities: it's that unless you live there, you will be easily identifiable as a tourist, and 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 pouch can consider an alternative solution. Many travel wear manufacturers — Tilley and ScotteVest 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, you must avoid the temptation to access such convenient hiding places in public (less likely with an under-clothing pouch). As for me, I prefer a proper security pouch.
Thoughts on Wallets: Once you've safely stashed your passport, visas, vaccination certificates, driver's license, 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. It's not unlike overpacking suitcases, although the motivations may not be identical. 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.
Finally, in these times of increased airport security, with official/officious folks checking our identification/tickets/receipts/whatever at every turn, some travellers 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, transfer the important items to your security pouch. Easy TravelAir makes a nice version of these airport pouches.
Click here for the final section of the ...
A one-page checklist version is available here.