Packing List
Health
I'm not legally qualified to dispense medical advice; what follows are personal opinions based on my own experience. For a more comprehensive and authoritative view of medical issues as they pertain to travelling, refer to Dr. Stephen Bezruchka's excellent (and wonderfully portable) The Pocket Doctor.
toilet paper, water squirt bottle, antibacterial wipes
You can't depend on finding toilet paper everywhere. You can't even depend on finding recognizable toilets everywhere. The minute you leave "western" civilization, you will quickly discover that considerably more than half the world uses some variant of the squat toilet. These can range from quite modern fixtures to what amounts to an open pit (when on safari in some desert, you likely won't find toilets at all). If you're unfamiliar with the use of squat toilets, your first encounter can be a daunting one indeed! Advance practice is highly recommended.
Briefly, pants are rolled up to the knees, and the upper part, along with underpants, lowered to the knees (you don't want them around your ankles!). Items capable of falling are best removed beforehand; retrieving them will likely be impractical. You squat by first bending the upper part of your body forward (to maintain balance), then lower yourself by bending your legs, coming to rest quite naturally (and comfortably) on your haunches and legs. Reverse to rise.
Aim is more important in squat toilets than in sit toilets, so don't fire indiscriminately.
Cleaning up in most places will likely not involve toilet paper, unless you brought your own. This is done using a (probably nearby) container to bring water to your waiting left hand, which will splash it on the appropriate areas. A better (more effective, more sanitary) solution is to bring along your own small plastic water bottle with a squirt top, able to direct a stream of water in the desired direction.
Washing one's hands when finished takes on a new importance; locals may well do this with water only, so you might consider bringing your own soap. Alternatives to soap include hand wipes treated with antibacterial solution (ensure that "Ethyl Alcohol 62%" is one of the active ingredients; one good brand is Purell Sanitizing Hand Towels with Moisturizers), and liquid antibacterial cleansers (often available in travel-sized containers), though the latter introduce the challenge of transporting liquids. There is some dispute among experts about the long-term value of antibacterial products like these (and none of them works on viruses anyway); all agree, however, that frequent washing is more important than the choice of ingredient to abet same. Unfortunately, most people wash their hands in a perfunctory fashion that accomplishes little. It should be done, with soap or the equivalent (which suspends the microbes such that they can be washed away), for at least 15–20 seconds, using warm (or cool, but not hot) water, and should include the backs of the hands, the wrists, and under the fingernails.
And now you know why the social use of the left hand is considered impolite in many parts of the world (it's also why punishment in some countries involves cutting off that hand). If you do use paper, do not deposit it in the toilet (unless there's no plumbing involved), or you will probably clog the pipes. There is likely to be a wastebasket handy: the soiled paper goes there. Yes, really.
There's much more to this topic than mentioned here, as there is truly an amazing variety of toilets to be found in the world. Read "Going Abroad" if you really want to expand your horizons.
(chemical or mechanical) water purifier
If you're travelling anyplace where sanitation is even the least bit questionable, this is a must. The problem, and its several solutions, are too diverse to cover here, but you really should learn about this topic: Rob Sangster's book is a reasonable (and current) reference, and Travel Medicine, Inc. sells a variety of purifiers. Bottled water can be expensive, and not always available (or even trustworthy; in this respect, bottled carbonated water is safer). Chemical additives taste bad, and may have long-term health implications. Mechanical (filter) purifiers are bulky, heavy, and most don't remove viruses.
The next best choice is probably the venerable 2–3 drops of iodine in a liter (quart) of water, left to stand for 30 minutes. Yes, there's a bit of a chemical taste, but there is with chlorine-based additives as well, and they don't kill giardia. You can pretty much remove both the taste and the colour by adding 50 mg of vitamin C (ascorbic acid) to the water, after it has stood for the treatment period; a flavoured beverage mix containing vitamin C is fine for this. Be aware that iodine stains terribly, and runs like crazy to boot, so pack it accordingly; if your iodine container ever leaks inside your bag, you'll never forget it! You can find de-coloured iodine in the U.S. and some other places, and the regular stuff at pretty much any pharmacy in the known world.
Popular wisdom states that iodine should not be ingested on a long-term basis (more than six months), but I have never seen any reputable data that confirms this. On the contrary, a study of the previously healthy inmates of three Florida prisons who consumed water disinfected with 0.5 to 1.0 PPM iodine for 15 years showed no effects on health or thyroid function. Of 101 infants born to prisoners drinking the water for 122–270 days, none showed detectable thyroid enlargement. Nonetheless, those with thyroid problems might consider alternative solutions.
If you're planning extended travels in areas where drinking water is a problem, consider a couple of the more recent high-tech approaches. One, as used by the U.S. military, is MSR's MIOX® Purifier (shown at right). This is essentially a miniaturized municipal water treatment plant that fits in your pocket. It operates by converting a brine solution to a mixed-oxidant (chlorine) solution via electrolysis. The purifier utilizes two 3V lithium camera batteries (CR123), common salt, and a tiny electrolytic cell; it will treat approximately 200 liters of water on one set of batteries. Using the MIOX is not a trivial process, with four hours required for complete treatment.
The other technology, embodied in Hydro-Photon's SteriPEN®, makes use of ultaviolet light to kill bacteria, viruses, and parasitic protozoa. This is a fast (under two minutes) and extremely convenient method, but because it requires light transmission, will not work on heavily clouded water; in such cases the water must first be allowed to settle, or filtered (this need not be onerous, and can usually be accomplished with a coffee filter, or even a bandanna; the company also sells a four-micron screen pre-filter that fits most wide mouth water bottles). A smaller/lighter/costlier model is available, but I prefer the "Classic" (shown at left), which uses AA batteries (the "Adventurer" and identical-except-for-colour "Traveler" models use CR123s).
Two final comments: avoid ice (which cannot be assumed sanitary) in questionable environments, and remember to use safe water when brushing your teeth!
analgesic of choice
Aspirin, acetaminophen [paracetamol], ibuprofen, naproxen sodium, ketoprofen, or whatever, are best (though not most efficiently) carried in their original packaging, especially if you look somewhat "counterculture", and/or expect to meet suspicious border inspectors. Ibuprofen is a good choice if it works for you pain-wise (it doesn't for me), as it also reduces swelling in muscles and tendons, a common byproduct of too much unfamiliar walking.
diarrhea treatment of choice
Studies show that 35% of all international travellers (and 50% of those visiting developing countries in Latin America, Africa, the Middle East, and Asia) develop "traveller's diarrhea" (TD). So there's a pretty good chance that this will be a part of your experience at some point, making it helpful to know something about its causes, prevention, and treatment. Several infectious agents, including bacteria, parasites, and viruses, can cause TD. While parasitic TD can occur 7–14 days after exposure, most shows up some time from a few hours to three days. In up to half of all cases, the exact cause is unknown. The most common bacterial cause of TD (20–50% of cases) is Escherichia coli (E. coli), found in soil, water, and milk. Diarrhea caused by E. coli is generally explosive, non-bloody, and accompanied by nausea, vomiting, cramps, and fever. Shigella is the next most frequent (15% of travellers) bacterial cause of dysentery, being common in countries experiencing natural disasters, socioeconomic upheaval, or other conditions making it difficult to procure sanitary food and water. Shigella infections produce bloody and mucus-laden diarrhea, along with fever, cramps, and exhaustion; such severe consequences are usually labelled dysentery. Next in line (2% of cases) is TD caused by the parasites Giardia and Entamoeba histolytica, found in contaminated water. Giardia can cause chronic diarrhea lasting for several weeks, along with vague pain, weight loss, bloating, and fatigue. And there are other bacteria (Salmonella, Campylobacter), parasites (Cryptosporidiosis, Cyclospora), and (less commonly) intestinal viruses like rotavirus. As you might imagine, dealing effectively with all these possibilities is not an easy matter.
Prevention should be the first line of defense. Don't ingest (or swim in) contaminated water; see the above discussion of this topic. With respect to food, the traditional traveller's adage ("If you can't cook it, boil it, or peel it, then forget it!") is still valid, but doesn't go far enough (in the light of current research). Heated food should be hot to the touch, and eaten promptly. Be particularly cautious of food from street vendors. Peel your own fresh fruits and vegetables (be especially wary of things like lettuce and sliced fruit, which may have been washed in contaminated water). Avoid dairy products and raw or undercooked meat and fish.
Pepto-Bismol (two tablets four times daily before and during international travel, but not for more than three weeks) can help prevent many cases of TD. In common with aspirin (which shares its active ingredient salicylate), its use should not be indiscriminate: many medications interfere with salicylate; pregnant women, people allergic to aspirin, and those with ulcers, other bleeding disorders, or gout, should first consult a physician; children under three, and children and adolescents with flu or chicken pox, should not take it. Possible (though rare) side effects of Pepto-Bismol include ringing in the ears, and black stools & tongue.
The two most popular prescription drugs for dealing with TD are Lomotil [diphenoxylate atrophine], which essentially blocks your plumbing for a day or two, and Cipro [ciprofloxacin hydrochloride], an antibiotic to kill the bugs that are causing the problem. As with any prescription drug, discuss the use of these with your doctor before taking them or administering them to others (especially children). The indiscriminate use of antibiotics is causing a lot of problems in the world. In some places (e.g., India and Thailand) bacteria have become quite resistant to Cipro; Zithromax [azithromycin] is currently proving an effective replacement, and may now be the best antibiotic treatment for TD.
Be aware that antimotility drugs such as Lomotil (and Imodium [loperamide], an over-the-counter alternative that works better for some people) do not prevent diarrhea (in fact, they may even increase the risk for it), but they provide prompt though temporary symptomatic relief by reducing muscle spasms in the gastrointestinal tract. Don't use them unless access to a toilet is limited (such as during a long bus ride). And never use them for more than 48 hours.
Rice, wheat, potatoes, corn, and chicken are foods known to slow the effects of TD.
As a rule, letting nature take its course is the preferred approach, abetted with plenty of fluid replacement: drink lots of pure water. In more severe dehydration cases, oral rehydration solutions (ORS) — incorporating potassium, sodium, & calcium supplementation — may be more effective. Pre-mixed, packaged ORS (just add water) can be obtained at shops selling hiking supplies; the solutions can even be mixed yourself, if you have the correct ingredients available. It's important to understand that the prevention of dehydration is the most important consideration: in severe cases, dehydration can be life threatening, particularly in children. Agitation may be an early symptom; severe indications include listlessness and a weak pulse, and are causes for serious concern.
infection treatment of choice
Another prescription drug, Keflex [cephalexin] is one of the cephalosporin antibiotics. It can be used for a wide variety of bacterial infections, including those of the respiratory tract, urinary tract, skin, and ear. Again, consult your doctor for details. Non-prescription triple-antibiotic ointment [bacitracin + neomycin + polymyxin B] — sold as Neosporin, Mycitracin, Topisporin, etc. — will help keep simple cuts and scrapes from becoming infected (those allergic to neomycin can substitute double-antibiotic ointment, such as Polysporin).
malaria tablets
A must in many parts of the world.
insect repellent, mosquito net
Even if you're not trekking in places like central Australia (with flies so thick that they have been known to drive horses literally mad), a lightweight, over-the-head mosquito net can save you from hotels whose ideas of adequate screening don't coincide with your own. In many areas of the world, you'll want a net (such as those available from Long Road Travel Supplies) large enough to cover your entire sleeping area. Spraying a net (and your clothing, for that matter) with permethrin will increase its bug-repelling ability. For your skin, you need something containing either DEET [n,n-diethyl-3-methylbenzamide] or picaridin [(RS)-sec-butyl 2-(2-hydroxyethyl)piperidine-1-carboxylate]; the higher the concentration, the more effective. Much enthusiastic advertising (and popular "wisdom") to the contrary, these are the only insect repellents that have proven effective in controlled, objective tests.
If you're concerned about getting bitten/stung by any of the flying, crawling, & swimming creatures capable of doing so, consider carrying a small Aspivenin® pump. 
Used by the French Army, Médicins sans Frontières, UNICEF, Wildlife Conservation International, etc., this clever mechanical device (developed by André Emerit, a French inventor who died in 1997) visibly extracts venoms without damaging the surrounding skin. You can learn more about it, and order one online for about $20, from Aspivenin (France) or from Flairpath Marketing (UK). Sawyer Products sells what appears to be a cheaper (in the U.S.) copy of this device: the Sawyer Extractor.
Cortaid Maximum Strength anti-inflammatory cream (containing 1% hydrocortisone [cortisol, a corticosteroid hormone], the highest non-prescription concentration available in the U.S.) will sooth bug bites, allergic rashes, eczema, psoriasis, & certain other inflammatory skin conditions.
sunscreen, lip balm
Malignant melanoma can be severely depressing, so heed the Australian sun protection slogan: Slip (on a shirt) — Slop (on some sunscreen) — Slap (on a hat)! A good non-liquid SPF 30 sunscreen is Neutrogena® Healthy Defense® Oil-Free Sunblock Stick. Be aware that sunscreens should be reapplied every two hours, and those more than 2–3 years old are no longer effective.
When using both sunscreen and insect repellant, remember that the former work by penetrating the skin to create a barrier, while repellents work by evaporation, so apply the sunscreen first. Waiting 15–30 minutes between the two applications will prove even more effective, especially as there is some evidence that DEET (likely because it is a solvent) reduces the effectiveness of sunscreen by about 40% (this also suggests a more liberal application of sunscreen than usual). It's possible to find products containing a mixture of DEET and sunscreen, but many researchers question their efficacy. Separate products are preferable in any case, as they make it possible to reapply sunscreen without the DEET.
Both sunscreens and decent lip balms are hard to find in many areas of the world, so bring enough to last you. Because most lip balms contain waxy ingredients, they can also be used to lubricate zippers, and even give your shoes a quick shine!
tweezers
Although they have cosmetic uses as well, a good pair of high-quality tweezers can be invaluable should you pick up a sliver or need to remove a tick or insect stinger of some kind. 
The very best tweezers available (stainless steel Uncle Bill's Sliver Gripper, pictured at left) are inexpensive, incredibly effective, weigh practically nothing, and carry a lifetime warranty. These are available with either a stainless keychain clip (which protects the points) or a small plastic vial; I find the clip more convenient, and easier to clean.
When General Norman Schwarzkopf discovered these (during the Desert Storm campaign), he wrote, "I have never had a pair of tweezers in my life that was worth a damn. Now I do."
bandages, moleskin (other first aid supplies?)
Lots of walking can lead to chafing, which is easily remedied if you have a bit of moleskin, and a potentially serious problem otherwise. In bandages, I like the cloth butterfly-shaped style (designed for fingers/knuckles): they can handle surprisingly drastic cuts, and are easily trimmed down to smaller sizes as needed.
menstrual, contraceptive, and/or prophylactic supplies
Women travelling for extended periods of time might want to give thought to alternative forms of menstrual supplies. Tampons, for example, are difficult to find in many countries, and more likely to be "toxic shock" specials. Travel- (and environment-) friendly alternatives include menstrual cups (such as the DivaCup, Keeper, and Mooncup), which sell for around $35 and last for ten years, and reusable cotton pads (such as GladRags and Lunapads). Some women make effective use of sea sponges, but this approach raises enough concerns that I am somewhat hesitate to recommend it.
Another approach to menstrual "management" during travel is based on a surprisingly little-known fact: Medically, it is quite safe to suspend menstruation for many months (even years, although I can't recommend this), using regular (constant-dosage) contraceptive pills — not the triphasic versions, which vary in hormone dosage over the three-week usage period. This is ideal for extended travelling, especially in more remote areas. I don't advocate taking hormones for this purpose alone, but a woman using them anyway can easily reap this particular benefit. Simply take the pills continuously (i.e., no breaks or "week off" sugar pills, which produce the drop in hormone level that triggers the period). It is, of course, extremely important to see your doctor first (to confirm that I didn't just make this up), and to have been taking the pill for at least several months before travelling (to check for any undesired side-effects, such as clotting or blood pressure problems).
Condoms brought from home are less likely to break, and more likely to be available; remember, the diseases most often caught during foreign travels are sexually transmitted ones. (And although some seem not to understand this, being "on the pill" does not stop transmission of STDs.) An unlubricated condom also makes a useful rain protector for cell phones & other small items, an emergency water bottle (when inserted in a sock to help hold the shape), and even — when its ends are knotted together — a makeshift "rubber" band.
vitamins, necessary medications
If you need to carry any prescription drug, be sure to bring along a copy of its prescription, both to replenish your supply in an emergency, and to verify your need for something that might otherwise be illegal in a particular destination country. The drug should be listed by brand and generic name, and include the dosage. If you have a serious condition, a letter from your doctor (translated into the local language) is a wise precaution. Carry more medicine than you expect to need, especially if travelling to places with inferior medical standards.
Have your prescription filled in tablet form when possible, as capsules and liquids are less stable, particularly in hot, humid climates.
If you're diabetic, review the National Institutes of Health's suggestions on travel with diabetes.
Avoid buying prescription medicines from other than known, trusted sources, especially in developing countries, where local pharmacies will compound drugs themselves, perhaps using something like sulfa (to which you may be allergic) as a filler. Counterfeit drugs are also a problem (as are expired drugs "dumped" from the U.S.), particularly in Mexico, Thailand, South America (except Argentina & Brazil), Africa, and Hong Kong.
Also, be suspicious of the sterility of products in many parts of the world. If you require syringes for injectable medicines, consider bringing your own (or sterilizing what you are able to buy).
Those prone to motion sickness should note that many common medications (both over-the-counter and prescription) used to prevent same have a mild sedative effect, so can often do double duty as sleeping pills.
(collapsible) plastic water bottle & cup
Drink plenty of clean water, like your mom told you (or should have), especially in hot weather. How to know if you're drinking enough? Check your urine: if it's not sufficiently clear to read through, you're not properly hydrated. (If you're severely dehydrated, you may need more than just water; refer to the discussion under "diarrhea treatment", above.)
Getting enough to drink isn't always easy, though, depending on where you are at the moment (like a plane, or other than an urban area of a developed country). So carry your own, in your daypack. A recent innovation is the Platypus® collapsible water bottle, which I have found to be surprisingly rugged, solving a longstanding problem of efficient water storage.
A cup is often useful, and can also be used for cleansing purposes in squat toilets. The best collapsible version I've found is the plastic Swedish FoldaCup (Magellan's carries them), standard issue for the Swedish and Norwegian military; it's lightweight (one ounce), durable, stable, leakproof, and will fit in your pocket. If you're planning to make extensive use of an immersion heater, you may want something less likely to melt (and also, alas, a bit more bulky, though if you choose a material like titanium, it won't be any heavier).
dark glasses, retainer, case
Sunglasses will save your sight for your retirement years (get good ones, with 100% UVA and UVB protection). The eyewear retainer ("Chums" is a good brand) and case may save your glasses.
lens cleaning cloth/supplies, copy of prescription (spare glasses?)
If you wear prescription lenses, and don't carry spare spectacles (or even if you do), a copy of your current prescription could prove invaluable. Cleaning supplies are of particular importance to contact lens users.
earplugs (eye mask?)
Earplugs can be extremely useful, not only at rock concerts (to save your hearing for your retirement years), but in situations where you're trying to get some sleep, and the party next door (or on the street) isn't. In noisy third world cities, and when you're trying to sleep on airplanes, these are pretty much a necessity. Best are the disposable foam types that you compress by rolling between your fingers; they expand in the ear canal. "Disposable" means that you don't keep them forever, but if you wash them after each use, they will last for quite a while.
The top brand is the Howard Leight Max (shown at left), with a superb 33dB noise reduction rating (NRR). If you'd prefer to buy in smaller quantities, consider AOSafety's E-A-R Plugs: they're a bit less convenient, and not quite as effective as the Leight product, but still much better than most of what's out there.
State-of-the-art earplugs aren't of much help if you don't know how to use them properly; there's more to it than simply jamming them in your ears. The video on the right explains what you need to know.
If you're planning to sleep on long flights, you may also find an eye mask helpful (Eagle Creek's "Comfort Eye Shade" is an effective, slim design that eliminates pressure on your eyelids).
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