Foreign Entry and Exit Requirements and Visa Courier Services
U.S. Department of State
The International Travel page on the U.S. Department of State's website is a great starting point to learn country specific entry/exit requirements for U.S. citizens traveling abroad for travel/business.
Zierer Visa Service
After checking visa requirements for a specific country, you may decide you want to use a visa courier service to expedite the process. Zierer Visa Service (ZVS) is a reputable visa courier service.
Travisa Visa Service
Travisa is another reputable visa courier service. They also have other international travel information.