From Baccus Miniatures comes Gettysburg At The Gallop, a set of free wargames rules for the American Civil War in 6mm.
Rules: American Civil War
Command Combat Civil War: Bull Run
Command Combat Civil War: Bull Run is both a rules set and a scenario for playing the American Civil War.
Battle for The Bliss Farm ACW Rules
Battle for the Bliss Farm is a scenario and set of free wargames rules for the American Civil War.
1862 Campaign Rules
1862 is a set of American Civil War Campaign Rules designed for Johnny Reb, but which could be used with any miniatures rules set.
Then Holler! Tactical Miniatures Rules For The Civil War
Then Holler is a new set of free wargames rules for game set in the American Civil War.