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.
The DFW Grognards have published a set of ACW Campaign rules.
Give Them Cold Steel is a set of free wargames rules for the American Civil War.