There are some countries that will honor a valid U.S. issued driver’s license, however, there are others that will also require an International Driving Permit (IDP). It's best to check with the tourist board for the country you're visiting before you to to get the latest information on exactly what you need to drive in another country.
You can apply for an IDP at your local American Automobile Association (AAA) office or via their website, at http://www.aaa.com/vacation/idpf.html.
You can check the embassy of the country you plan to visit, for specific requirements: http://www.usembassy.gov/. Keep in mind, however, that while a specific country’s government may not require an IDP, certain car rental companies located in that country, such as Hertz, may still require one.
For more information on driving abroad, please visit the following US State Department site: http://travel.state.gov/content/passports/en/go/safety/driving.html.