You will always need identification when you travel. You will almost always need a passport.You may need a travel visa in some countries. As a US citizen, you will (almost) always need a work visa to work legally in other countries - this requirement may vary if you have a US employer. You will seldom need, but may want to carry, travel immunization records. You may want, but *may* not need if you are renting a car abroad, an international driver's license. Although you can currently use a birth certificate and driver's license as id.
Passports are $97 through a US post office if you're over 16 ($70 otherwise) and are valid for ten years, and function as universal id. You can also use a passport to meet INS employment requirements in the US.
Travel Documents