Mark Ainsworth offers a set of simple solo wargames rules for games of the 7th Cavalry against Plains Indians.
Tombstone is a free RPG set in the wild west. It’s designed to be rules-light, and has quick play character creation to get you started.
Black Hat, White Hat is a set of free wargames rules for Wild West skirmishes.
Dusty Trails is a set of free rules for wild west gunfights.
Blaze of Glory is a set of free wargames rules for Wild West skirmishes.