ID

Drivers License or other form of "airline approved" photo ID - Necessary to board airplane.

Student ID Card - Great way to get museum, movie and other discounts (if you're a student).

Passport/Visa- For visa requirements, check with the embassy for the country you wish to visit.

Business Cards - Great way to give contact information to the friends you   make while on the road.

Just in case

Key phone numbers (airlines, hotels, rental car, etc.)

Reservation confirmation numbers

Make photocopies of important documentation (i.e. ID, credit cards) - in   case of loss or theft.

Medical

Certificate of vaccination - For "exotic" locations check with the destination embassy for requirements.

Money

ATM/Credit Cards/ Cash

Traveler's checks

Telephone calling card

Driving and Insurance

Drivers License - U.S. Car rental companies require a valid driver's license (no temporary permits or photocopies) and a credit card at pick up to rent a car.

Insurance - Rental car insurance may be covered by your existing auto insurance policy or by the credit card you use to pay for the rental. Check before you choose the expensive insurance offered by the rental car company.

Maps: Make sure you have a map of the area you are visiting. IgoUgo provides MapQuest.com maps and directions under Travel Planning.

Friends and Family

Take addresses and phone numbers of friends and family so you can send postcards from the road and/or contact them in case of emergency.