Under The Shade Of The Trees is a set of free wargames rules for the American Civil War. They're designed as "fast play" and suggest using standard Johnny Reb or Fire and Fury basing.
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.
Mighty Mean Fowt Fights is a set of free wargames rules for American Civil War battle games played on a square grid.
Two Flags One Nation are a set of rules for do-it-yourself American Civil War board games. The author explains how he generates maps and block pieces with his computer.