Honor and Glory is a set of rules for the American Civil War for games where the smallest tactical unit is a regiment, which is represented by four bases. It’s playable in nearly all figure scales. Originally available on the Yahoo Groups, I have archived the files there because Yahoo Groups is shutting down.
Bob Bergman has written a set of free wargames rules for the American Civil War called Seeing The Elephant. Designed for two different regimental and a brigade scale, the rules use a card activation mechanism, combined with a Morale Check. Firing is done by adding Fire Factors and rolling a die; Melee by figuring odds and rolling a die. In all, a comprehensive, well-thought out set of rules.