The Civil War Soldiers and Sailors System is actually several databases in one and can be an excellent free genealogy resource for filling in military details. The soldier database provides the name, regiment name, side fought on (Union or Confederate), company and the person's rank going in and coming out of the war. If available, it also gives you alternate names for the individual and any notes. Finally, it gives you the film number so you can locate the index card for yourself or simply use the information in your source citations.
The sailors database currently holds only records of Union African-American sailors. It gives quite a bit of information including name, birthplace, age, complexion, occupation, height, naval service information and muster information.
The cemeteries database is a work in progress. It currently only has listings for those buried in Poplar Grove National Cemetery. Results include name, company, unit, rank, state, military organization, date of death, original burial place, gravestone number and a photograph of the headstone. Some also include other comments.
The prisoners database has records from Fort McHenry (Confederate prisoners) and Andersonville (Union prisoners). Fort McHenry records give you name, type of prisoner, residence (if known), date of registration, date of disposition and place of disposition. Andersonville records offer more detail including name, side, unit name, regiment, state, function, company, rank, type, capture date and site, alternate names and remarks.
The medal of honor database rounds out the collection. It gives you the name, rank, citation state, unit, company, citation place, entry place and state, birth place and state, date the citation was issued and information on why the citation was issued.