Richard Bradley’s State of War ACW Skirmish Rules are available at the Tyneside Wargames Club site.
By Jeff Peach & Les Booth offer a set of free wargames rules for ACW River and Coastal Warfare
Here’s a set of club developed American Civil War rules.
From Baccus Miniatures comes Gettysburg At The Gallop, a set of free wargames rules for the American Civil War in 6mm.
Command Combat Civil War: Bull Run is both a rules set and a scenario for playing the American Civil War.