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.
Tin Army In The Civil War is a set of free wargames rules for the American Civil War by Ross Mcfarlane. The author writes:
This is a simple set of wargame rules aimed at Division or Corps sized battles. They assume that low level tactics are being handled by regimental officers and their Brigadiers and by the dice. The game may be played on grid with 1 unit per grid area otherwise all distances are roughly equal to the frontage of a deployed infantry unit or roughly 150 yards. Units within that distance and directly in front of, behind or beside each other are deemed to be adjacent. Turns represent a variable length of time and overlap the previous and following enemy turns. 12 to 15 turns each is a good medium length game with a simple scenario and a dozen or so units per side.
The DFW Grognards have published a set of ACW Campaign rules.