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.
From the Naval Wargames Association comes Ironclad, a set of free wargames rules for ACW ironclads.
Howlin Mobs are a simple set (five pages) of ACW rules for big battles.
Ross McFarlaine offers Simple ACW Army Level Hex Wargame rules