Caveat lector: this list will most benefit people like me: young men from the United States. It can be adapted for women or other countries, of course.
The below items supplied me for ten weeks' travel in Europe.
People commonly advise travelers to "pack lightly," but don't give any concrete information about how to trim down what you carry. Hopefully this list can help you plan.
The two pieces of luggage used were a "day pack" and matching soft luggage bag. At the time, both bags could be carried on an international airline, so I didn't have to worry about checking bags.
These two pieces of luggage could be zipped together into one fairly large item. If I ever needed to claim I was only carrying one bag, I'd zip them up. When I needed to be more mobile, I'd take them apart.
The guiding philosophy for this list was to justify every item, prepare for all weather, and be willing to lose any and every item. I only lost a few things, but it was nice not to be too attached to them.
Update: It's been suggested that this list is too much stuff to carry around, so I've tagged each item I perceive to be optional. Of course, people travel at different comfort levels: some people might feel best with just the clothes on their backs.
Here's what I carried all the time:
Optional. Most hostels and hotels will give you a key to your room. I'm used to carrying keys on a caribeaner. After I lost the Nalgene holder, the caribeaner was a nice way to hook the bottle to my day pack. Caribeaners also come in handy for hooking arbitrary stuff together.
- comfortable boots
You only want one pair of shoes. I used "Rockports," which look somewhat dressy but hold up to rocks and bend when needed.
- change purse
Optional. Europeans use coins more effectively than the USA. You will need easy access to coins.
I wear glasses. I'd actually recommend glasses over contact lenses because you won't have to worry about running out of lenses, you might lose your lens solution, and of course you don't have to touch your eye to put on glasses.
- large wallet
European money gets bigger as it becomes more valuable: a 20 pound note is bigger than a 10 pound note is bigger than a 5 pound note, and same for euros. USA wallets that barely fit USA dollars will not be sufficient.
- money belt
Optional. Tourists get robbed. If you get robbed, you need to be able to recover. I carried my passport, traveler's checks, and large bills in a money pouch tucked in my pants. Rock on!
- swiss army knife
Don't take. I don't actually remember carrying a swiss army knife, but hey, there it is, written down in the journal. They're probably useful; remember to have tweezers in case of splinters. Warning: you can no longer take knives with you on planes. Leave this item at home.
Optional. I try not to wear watches, but when you're traveling you need to know when to meet public transportation.
Here's what stayed in my day pack:
- Camera + bag + ISA 200 film
Optional. Here's my heaviest item: a Nikkormat circa 1970s SLR that probably weighs five pounds. I carried it in its own padded bag inside the day pack. Before I left I agreed it would be OK if I lost the camera. I carried 23 rolls of film: 4 in the camera bag and the rest in the luggage. X-Rays don't do much damage to film, by the way, unless you're using ISA 3200 film or they get X-Rayed a lot. ISA 200 is good for daylight shots, which should comprise at least eight percent of your photography. I didn't have a flash, but I did have a f/1.4 50mm lens, which means I could take dim pictures at nighttime.
- emergency two snack bars
Optional. If you carry one snack bar, you will never eat it. If you carry two, you'll eat the first one, and then justify eating the second one. Take bars that can last the length of your trip without grossing you out.
- Journal + two pens + pencil
Optional. I put photocopies of all my documents in the journal, put a monthly calendar in the back with one-word summaries for each day, numbered all the pages, and added a table of contents. If you're on public transit for a bunch of hours--even on the flight over!--you may find yourself organizing, too.
- Let's Go Europe
Optional. Stupidly I tore out all the countries I didn't think I'd go to. Well, turns out I also went to the Czech Republic. Let's Go will keep you entertained, tell you what all the other tourists would tell you, and can get you started in new towns.
- Marmot Precip rain jacket
Mostly brands don't matter to me, but the Marmot Precip was ranked number one for rain jackets by Backpacker magazine and it's the cheapest in its class. You open the "pit zips" for hot weather; you tighten the wrist and belt straps when it's windy or cold. Pick one up, and you're done worrying about weather (on the upper half of your body).
- Nalgene bottle with velcro Nalgene pouch
Optional. Water is cheap. I filled up my Nalgene bottle at the hostels and didn't have to worry about buying water at tourist traps. The velcro sleeve was a good idea, too, because it stinks to take off your pack just for a quick sip.
Update: I've been warned that this is bad advice--that you shouldn't drink tap water in many parts of Europe because you may get diarrhea. filoraene says, "Drinking tap water has the potential of ruining your holiday, so DON'T, or ask a local. They will give you an offended look if it is drinkable, though. Buy water bottles at the super market. Another piece of advice: cans of coke are full of precious energy and water. They can really keep you going while walking."
Rolled Up Stuff
Everything in this section was first laid on top of one another, then tightly rolled and put in the luggage. Rolling keeps the clothes from getting too wrinkled and makes everything fit tight.
- dress jacket
Optional. If you're going to a classy place, wear the khaki pants, boots (of course), fishing shirt, and dress jacket. The dress jacket can also be a nice way to "spice up" your outfit if you're around the same people for a long period.
- fishing shirt
The fishing shirt is a utilitarian travel garment. It's lame. The back side of the shirt is vented, so you can stay comfortable when it's warm. The sleeves roll up easily. If you wear a jacket, you're suddenly wearing a button-up shirt. The nylon fabric dries easily.
- polo shirt
Polo shirts are dress casual and can be worn in most conditions.
- short sleeve shirt
You need three shirts. I bought a comfortable but tight-fitting short sleeve shirt in Spain to stick out a bit less.
- four pr boxers
The number of pairs of underwear dictates the number of days you can go without laundry. Please don't be gross.
- khaki pants
Sometimes you need to look presentable at a restaurant, so take one nice pair of pants.
- two pr ripstop pants
The pants with zippers at the knee, so they can serve as shorts or pants. They're made of nylon so they dry quickly and are more comfortable in the rain than cotton.
- swimsuit shorts
Optional. Yes, pretending that a swimsuit is a pair of shorts is geek chic. But you're trying to cut down on baggage. Find a nice black baggy pair of swim shorts.
- three pr wool socks
Wool can get hot, but it provides a lot of padding for your feet. I wore wool socks in Scotland and Spain. I never got blisters, even when walking a twenty-two mile day up Mount Snowden.
Optional. Sometimes you need a towel. It's the most useful thing in the universe and it doesn't take up much space.
Optional. I can't make myself look presentable without a comb.
- Dr. Bronner's Magic Soap
Yes, it really is "18-in-1." Here's your face wash, deoderant, soap--even toothpaste. I used it for toothpaste for a month and a half before having to quit. Pick your scent carefully as you will smell like it for your whole trip. You can't buy replacement Dr. Bronner's in Europe, far as I can tell; I took the pint-sized bottle.
A must-have. If you're saying to yourself, "I don't floss. Skip this junk!," I would say (1) you can use floss for things besides your teeth and (2) you will eventually want to floss.
Optional. Good for not looking like a bum.
Optional. Dr. Bronner's is not good shampoo.
- Berlitz Guides
Optional. Berlitz Guides are supposed to bring you up to speed on a foreign language you've never heard. Suffice it to say, they don't work. I did learn a few things from the front eight or so pages, about how to say "please", "you're welcome," and such; perhaps you could photocopy those pages and put them in your journal so you don't have another tiny thing to lose.
Optional. Clothes dryers are cool and everything, but they're rarely seen in Europe. Instead, washing machines have ridiculously high RPMs to knock out the water and then you hang your clothes to dry. I used this tiny clothesline a few times before it broke; often the place you're staying will have a "heat room" or area for drying clothes.
- knife + spoon + fork
I ate a lot of groceries to save money, but except for in England the hostels didn't have any utensils. Carrying my own (plastic) set was helpful.
- plastic bags + container w/ lid
Optional. Bags are probably the most useful travel utility: liquids spill, foods need to be covered, stuff gets dirty or wet. The associated container is good for spare food or trinkets. One of my bags was a trash bag, which I used immediately.
- backup glasses
Optional. If you decided to take glasses, make sure to have a second pair so your trip isn't ruined if you lose your first pair. Glasses would be a pain and a half to get replaced in a foreign country.
- sleep sack
Optional. Actually, I shouldn't have taken a sleep sack, but some travel guides recommend one in case you hostel doesn't provide linens. I recommend getting a new hostel if you don't receive linens. Here's an opposing view of hostels without linens, from shimmer: "I've been to places in the past where you have had to pay extra for linens. They weren't much extra cost though. The places weren't awful, but were very cheap!"
So, I hope this list helps you prepare for your travels. It is possible to pack lightly; you'll thank yourself later!