North Against South is a set of free wargames rules for civil war battles. It's based on the boardgame Battle Cry by Richard Borg.
Mike Callan offers this set of free wargames rules for American Civil War Ironclads. The set originally was published in Wargames Illustrated 14.
Peter Hunt of the Hong Kong Society of Wargamers has written a set of articles about Naval Warfare in the American Civil War. This is, I think, a sadly neglected period for wargamers. I have a couple of dozen ships in 1/1200 scale, and the games have always been well received.
John Martin has written a set of free wargames rules for the American Civil War based on Games Workshop's Warmaster. I think that things have come full circle here, because Warmaster obviously took much of its inspiration from the seminal ACW rules set, "Fire and Fury." Not that GW would ever admit that. After all, according to the Evil Empire, miniature wargaming is the "Games Workshop Hobby", which precludes the acknowledgement of any other rules set or miniatures.