I was impressed with their collection of books and resources dealing with the Civil War, particularly the Confederacy. Their Confederate Corner contains a Roster of Confederate Soldiers containing of names and units. If your ancestor served from Virginia, you will be able to find the roster in the Virginia Regimental Series. There are also specific Northern Virginia regimental records on microfilm. Major Civil War compilations, such as Dyers Compendium can be found in this section of the library.
The library has an extensive collection of photographs, manuscripts and maps. Their rare book collection is the original collection of the Alexandria Library Company which was begun in the 1790s. If your ancestor happened to live in Alexandria, you will want to use the city directories along with film of building permits and other documents to determine exactly where they lived. There is a good chance the house is still standing.
I have added this library to the top of my list. They deserve another visit!