There are certain items that are worth their weight in gold if you are traveling internationally and many of them are likely not the first ones that come to mind.
When you're getting on a plane to cross an international border, the first thing you should put in your pocket is a ballpoint pen. Almost invariably, unless you're traveling within the EU, immigration and customs paperwork will demand your attention during the flight. To fill out said paperwork, that ballpoint pen in your pocket will come in handy. Note that I write that the pen should be in your pocket, and not your carry-on luggage. If your pen is not in your pocket, it's likely that your carry-on luggage will be stowed, you'll be seated in a middle or window seat, and the aircraft will be pitching up and down due to turbulence, all of which will prevent you from getting at your pen until well after you've landed. If you're planning on filling out your paperwork after you reach the airport terminal, I hope you enjoy long waits in line because you'll be at the back of a four-hundred eleven person queue as the 747 empties out and all the other passengers proceed directly to immigration. The next items to slip into your pocket are earplugs. Until you've used earplugs on a flight you probably don't realize just how loud an airliner is. The engines certainly make plenty of noise, but so does the air conditioning system and the air rushing by the fuselage. In addition, earplugs are reasonably effective at drowning out the noise of crying babies and small children. And don't worry about missing out on any of those supposedly important announcements from the cockpit or the stewards. The PA system on an airliner is turned up quite loud so that it can be heard over all the rushing air and engine noises; you won't have trouble hearing it with earplugs in. Finally, if you're flying on a military transport, earplugs are standard issue as those planes have no sound insulation. I recommend Howard Leight LaserLite earplugs, though others work nearly as well. Again, earplugs go in your pocket, not your carry-on luggage.
A Sharpie marker should also accompany you on your travel. This can travel in your carry-on. I don't recommend the ultra-fine point; the fine point is probably what you want. At worst, it can serve as a backup for your ballpoint pen in note-taking situations. At best, you'll find it invaluable for writing on packages you send back home through the mail or for writing your name on food that goes in a communal fridge if you stay at a hostel or backpackers.
By now, everyone knows about the useless but inflexible security rule that all liquids larger than three ounces are banned from carry-on luggage. This presents a problem if you want to bring home some olive oil or a small bottle of spirits from the country you are visiting. Hence, I like to carry a gallon-sized Ziploc or Glad plastic bag in my luggage. That way you can seal the bottle of liquid in the bag and place it in your checked luggage for the trip home. The gallon bags also come in handy if you find yourself with some leftovers you want to save from a meal you cooked in a hostel or backpackers.
Sleeping on an airliner is difficult at best due to the fact that you're forced to sleep in an upright position. Airlines used to routinely provide pillows and blankets to ease the discomfort somewhat, but now they don't bother (generally due to cost cutting). So, you're best off bringing your own pillow. I recommend an inflatable neck pillow (Eagle Creek makes a couple, as do others). The neck pillow fits around your neck and supports your head and neck no matter which way your head flops during flight. The advantage to an inflatable model over buckwheat or another pillow filling is that it can also be deflated make storage and transportation easier. I'll often see people with other travel pillows lugging them around the airport and it never looks fun. Meanwhile, an inflatable pillow is conveniently stored in my backpack and then transferred to the seatback pocket when I board the plane until I need it.
If you're easily bothered by ambient light while sleeping, a sleeping mask will make your life easier. Again, Eagle Creek and others make and sell these. You can use the mask both on the flight and while sleeping in foreign lands while trying to overcome jet lag. Some airlines will provide these on long overseas flights but the one you bring will be more comfortable and durable.
A small notebook often comes in handy for jotting down random bits of information that you don't want to lose. I prefer the 3"x5" spiral bound size but whatever is small enough to easily carry works well.
If you're purchasing a digital camera for travel, I suggest buying one that runs on AA batteries. You can use rechargeables to save money and the environment most of the time, but if they run out at an inopportune moment, you can always get a set of AAs from a corner store and get up and running again.
If you want to be easily identified at 100 yards as an American tourist, bring a baseball cap, preferably one that is well used. If you want to keep sunburn to a minimum and travel a bit more low-key at the same time, I suggest a hat with a brim that runs around the entire hat. This will keep your ears and, hopefully, your neck from suffering from the sun's rays.
These are all small items that will make your life a bit easier as you travel. You may not use every item every trip but if you need one and you already have it in your hands you'll likely never travel without it again.