2 pairs of pants (men/women)—Take quick-dry travel slacks (with secret pockets), not heavy, never-dry jeans; I always make one a pair convertible pants ( men/women) so I don't need to bring extra shorts.
1 pair of shorts with pockets or convertible pants (men/women)—European adults don’t often wear shorts (& most churches won't let you in with bare knees), but they’re good for hiking and, for men, as swimsuits (women: buy a swimsuit there if you find you need one—a great souvenir!).
1 long skirt or dress—The skimpiness at which your respectability will be questioned varies with the country, so hedge your bets with something long.
4 pairs of underwear (men/women)—They even make disposable underwear now.
4 bras or camisoles
4 pairs of socks (men/women)
3 T-Shirts (men/women)—Wear under long sleeves so the easily washed T-shirt will soak up all the sweat. Get quick-drying tops, not cotton.

2 long-sleeve shirts (men/women)—Button-down collared shirts are respectable for all occasions; travel ones have hidden pockets, sunblock, and easy washability.
Sweater (men/women)—Warm and dressy. Or you can do a...:
Jacket (men/women)—Only for fall to early spring (though a light rain jacket is always wise).
Wrap/Shawl—For covering bare shoulders (or improvising a below-knee skirt over shorts) to visit churches (it's a rule); also, for warmth on planes and cool evenings, and sun protection.
Long Underwear (men/women)—Only for winter trips to colder climes.
Good walking shoes (men/women)—No dress shoes, heels, flip-flops, or anything you can’t walk in all day for two weeks. (STURDY hiking or travel sandals—men/women—are also OK.)
Hat (men/women)—An all-around brim offers sun protection and rain protection (also: stylish). The Tilley Hat is the ultimate travel topper.
Belt—Those with a hidden zipper let you hide your passport photocopy and some emergency cash.

Toothbrush & small tube of toothpaste
Small soap & small shampoo—Bring the soap sliver from your bathtub to start; filch more from hotels as you go.
Razor & shaving cream—(Battery-op shavers are OK (electric razors just bring the hassle of electrical converters and adaptors).
Medicines—Prescriptions should be written in generic, chemical form (not brand name). Pill orginizers are handy.
Extra glasses/contacts— Count on losing them, and bring a hard glasses case. Also, bring enough saline solution to last (parts of the world sell it only in glass bottles).
First-aid kit—Take at least: a few Band-Aids, antiseptic ointment, moleskin (for blisters), aspirin, Dramamine or motion-sickness wristbands (I swear by Relief Bands, worth the outrageous price) hand lotion and lip balm (traveling promotes chapping), sunscreen, Pepto-Bismol (for indigestion and diarrhea; chewable, not liquid), and decongestant and antihistamines (for colds and unexpected allergies to local allergens).

Comb or flat brush
Laundry kit—To wash clothes on the go in your bathroom sink, you need travel detergent (biodegradable), a braided clothesline (the twists act as clothespins), and I suggest the truly remarkable stain eraser (perhaps I'm a slob, but I need them at least once per trip).
Towel—HG2G fans don't need to be told this, but a shammy-style camping towel or even small terrycloth towel is a lifesaver when confronted with Europe’s nonabsorbent, waffle-pressed jobbers.
Feminine hygiene products—You can buy tampons abroad, but take what you need with you, especially if you’re brand-loyal.
Condoms—US brands are safer.
Pocket-sized tissue packs—Invaluable for sudden spills, substitute napkins, bathroom emergencies, signaling surrender, and, if still clean enough, runny noses.

Guidebooks and phrase books
Journal and pens—You won’t remember it all half as well as you imagine.
Camera — Bring extra batteries. Tote it in a purse or mild-mannered daypack, not a “steal-me” professional camera bag. I like the new waterproof pocket cameras.
Memory Cards—Very expensive abroad (and you will fill them up fast!)
Tripod —I like the GorillaPod , with flexible legs so you you can wrap it around tree branches and other impromptu supports.
Tiny flashlight (I prefer a hands-free headlamp).
Travel alarm clock
Plug adaptors—For charging your digital camera, cell phone, etc.
Chargers/cables—I carry all my plugs, cables, and adaptors in a small toiletry bag.
Cell phone —Only bother bringing yours if a tri– or quad-band world phone with AT&T or T-Mobile (on the GSM standard used in most of the world). Otherwise, rent a cellphone. » more
Swiss Army Knife or Leatherman/multitool—Most useful features: screwdriver/can opener, blade, corkscrew (for picnics), tweezers, scissors, nail file. Note: The TSA has been debating allowing small knives in carry-ons again—though HAS NOT YET changed the rules. I'll keep you posted. For now, pack any knife in your checked luggage. (Official TSA regs)
Small bottle of water—Buy them as you go, but always have one. Outside of Europe and the North America (or in the backcountry), carry a bottle with a built-in purifier or get a UV sterilizer.
Sunscreen
Bug spray—Most of Italy has yet to discover window screens, so insect repellent is handy. Also useful: After Bite for the itch.

Luggage locks—Get enough for every zipper; make sure they're the kind approved for TSA use.
Sleep sack—An ultra-thin sleeping bag made of cotton or silk (sometimes called a "liner"); you need if you'll be camping or staying in hostels.
Tiny folding umbrella
Sunglasses
Pocket Sewing Kit—Trust me.
Duct Tape—Ditto.
Book or e-Reader—For long plane and train trips. Many hotels have rotating bookshelves to swamp out when you're done. [If an e-Reader, load up on titles before you leave, or get one with WiFi (so you can download outside the U.S) or one of the new Kindle 3Gs (which connect for free in 100 countries).]
Address list—Friends appreciate postcards at the time more than a slide show afterward.
Teensy binoculars—Maybe I'm getting old, but increasingly I find these useful for admiring distant frescoes, wildlife, landscapes, popes, etc.
Neck pillow - For the plane; long train rides. Some prefer the all-around-the neck style to the old horseshoe collar (this nifty model also converts to a small rectangular pillow). An eye mask is useful, too.
Noise-canceling headphones - The one seemingly silly travel gadget I actually use (it really does make flying less stressful, even if you don't sleep; also: easier to hear the movie). There are tons of models. I currently rock a JVC HANC250—1/3 the price of Bose; just as good.
Driver’s license & International Driver’s Permit—Only if you plan to rent a car.
Wallet—But keep important stuff in the moneybelt.
Money—Credit cards, ATM bankcard, traveler’s checks, and cash (euros if you got 'em and some emergency dollars).
Passport—You won't get very far without it.
Airline tickets—Crucial.

Tent—I cannot recommend strongly enough the REI Quarter Dome (the T1 model for solo trips; T3 to sleep 2–3). Inexpensive, lightweight, easy to use, and durable. Need more room? My Boy Scout troop uses the marginally heavier (but sturdier) 4-person REI Half Dome 4 Tent.
Sleeping bag - Either a Summer travel sack (55F+) or a 3-season bag (25+).

Sleeping bag liner—Silk liners pack teensy and work alone on hot nights, or add another 10° to your bag's rating when it's cold. (FYI: Fleece liners, while comfy, are bulky, and verboten at hostels—might house hitchhiking critters.)
Sleeping pad—Self-inflating pads are comfier and smaller; closed-cell foam pads are sturdier and cheaper.