1. An outlet converter. Make sure that all your electronic gadgets handle the higher number of Watts from European outlets.
2. Cash. Credit cards are not as standard in some parts of Europe yet. In many countries (Germany, Eastern Europe, Spain,…), you pay cash, especially for food and smaller towns. Either order it at the bank in your home country before or take it out of an ATM in your prospective country.
In Scandinavia, however, you can pay for almost anything via credit card.
3. Inform yourself about the cheapest option for cell phone usage: Ask your carrier at home how much it would cost to use it abroad, but also search for pages in your prospective country to see if there’s a cheaper option (such as a prepaid card). If you travel for months at a time, your US carrier can put your contract on hold in exchange for a small fee (+/- $10 per month).
4. Clothes according to the weather in your prospective country.
5. A small dictionary.
6. Comfortable shoes! Despite taking lots of public transportation because it’s easier and faster most of the time, you will walk a lot.