Remember to look for military records for ALL the male members of your family. Your many great grandfather may not have served in the American Revolution but if his brother did his records may tell you more about the family.
U.S. Department of Veterans Affairs now has a Gravesite Locator to National Cemeteries
Ancestry remains one of the best starting points for military research. If you don't have a subscription you might visit the local family history library (see the Archives & Libraries page).
One extremely helpful database at Ancestry is their U.S. Dept. of Veterans Affairs BIRLS File 1850-2010 which has over 8 million names. If you are having trouble finding the person and they lived late enough to have a social security number use the SS Death Index to locate his number. You can then put the number into BIRLS database and he should come right up.
Military Rosters - from Rootsweb (all wars) - currently has 111056 records and keeps growing by contributions
National Archives: Resources for Genealogists - this is where you go to order records. They dont' have a lot of online content but they do have a lot of good information and guides.
American Merchant Marines (database of all wars)