The US Department of Veterans Affairs has amassed an online database of the burial sites for more than five million veterans.
The Nationwide Gravesite Locator can help families, friends, and fellow veterans locate the cemeteries and plots for veterans from the Army, Navy, Marine Corps, and any other military service.
Records can be searched on last name (full name, or a 'begins with' search). You can search the US as a whole, or specific cemeteries.
Search results include name, rank, date of birth and death, war service, and burial location.
For overseas burials, use the databases maintained by the American Battle Monuments Commission, which include foreign burial records for the Mexican and Spanish-American Wars, the Civil War, World Wars I and II, the Korean War, and the War in Vietnam.
Don't forget to also check for your family history at NewspaperArchive and Newspapers.com. These are subscription databases, but they are among the most powerful research tools available for looking into family roots. And visit the main page of Free Genealogy Tools for more, umm, free genealogical tools.