One of the best web pages which links to a wide variety of free military web pages, is ShoeString Genealogy: Free U.S. Military Databases.
All in one location you can link to a variety of databases, such as The Veteran Ancestor Registry, U.S. National Cemetery Administration Nationwide Graveside Locator, 1820 U.S. Pension List and more. The nice thing about ShoeString Genealogy's link area is that each link has comments which supply more information.
The topics are general and then break down into various wars, beginning with King Philip's War (1675-1677). Under the Revolutionary War you will a variety of records, including the British, German and Loyalist Officers in the American Revolution, along with the Valley Forge Muster Roll and more.
The links continue through the Vietnam War and conclude with some miscellaneous databases. It is certainly a great place to explore and further your knowledge of the various types of military records. I hope that you hit pay dirt and find what you need in your search for military ancestors.