From the Naval Wargames Association comes Ironclad, a set of free wargames rules for ACW ironclads.
Rules: American Civil War
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!).
State of War ACW Skirmish Rules
Richard Bradley’s State of War ACW Skirmish Rules are available at the Tyneside Wargames Club site.