American Civil War Riverine Actions is a set of free wargames rules for ironclads, tin-clads, cotton-clads and all the fun and unique ships that plied the rivers of the United States during the ACW.
American Civil War Sh*t For Men Rules are a set of grand tactical Civil War rules for 15mm originally based on Napoleon's Battles, but now quite removed from the base.
Call To The Colors is a set of free wargames rules for gaming the American Civil War in 15mm.
The Revised Brom is a set of free wargames rules by Jay Ainsworth for wargames set in the American Civil War. They're based on an original set of Larry Brom rule (duh). It uses a typical Brom card activation system, a random movement distance mechanism, and simultaneous fire.
See The Elephant is a set of free wargames rules for the American Civil War played on a chess-like grid.