Give Them Cold Steel is a set of free wargames rules for the American Civil War.
Rules: American Civil War
Ironclad American Civil War Naval Rules
From the Naval Wargames Association comes Ironclad, a set of free wargames rules for ACW ironclads.
Howlin’ Mobs ACW Rules
Howlin Mobs are a simple set (five pages) of ACW rules for big battles.
Simple ACW Army Level Hex Wargame
Ross McFarlaine offers Simple ACW Army Level Hex Wargame rules
Steam Iron and Tin ACW Naval Rules
Steam Iron and Tin is a set of 15mm ACW naval rules that was distributed to people who attended the 1985 Origins Convention in Baltimore (I was at that convention!).