The Napnuts’ Chew Boon Longe has a set of free wargames rules for western gunfights.
Dark City Games’ Legends of the Untamed West is a tactical RPG for the 1800s American West. The system is based on the wonderful Fantasy Trip system that originally was published in the micro games format. If you’re familiar with the Fantasy Trip, you know that it makes for just as good a miniatures system as an RPG.
The Perfect Captain Yahoo Group has a variant for their Red Actions game for the Indian Wars called Crazy Horse’s Fury.
Fred Diamond offers a free and simple set of miniatures rules for playing pony wars — US Cavalry vs Indians.
Sandpit Showdown is a set of free wargames rules for backyard battles with army men in the Wild West and Civil War.