Mark Ainsworth offers a set of simple solo wargames rules for games of the 7th Cavalry against Plains Indians.
This Yahoo group has free wargames rules for recreating battles of the Indian Wars in the Old Northwest: the French and Indian War through the Blackhawk War.
Majestic 12 games has these demo rules for their Grand Fleets miniature wargame. My group has played these and really enjoys them. Majestic 12 explains:
Grand Fleets is a comprehensive miniatures game of dreadnought-era naval warfare. In addition to rules for everything from ships to aeroplanes, Zeppelins to submarines, the rulebook includes:
Data cards for over 120 ships, plus guidelines for converting any ship of the period to game values; Eight scenarios encompassing conflicts from the Sino-Japanese War of 1894 through the opening salvoes of World War Two, including several variants; Fifteen pages of weapon data, covering all major gun and torpedo models in use through 1939, over 200 aeroplanes, and more than 20 airships; and Optional rules for flotillas, night-fighting, smokescreens, and more!
Rorke’s Drift has captured the imagination as one of the greatest stands in history. Thousands of brave Zulu warriors versus a handful of stalwart British soldiers. What’s not to like? The battle is a staple for colonial wargamers. Steve Winter has these free wargames rules for Rorke’s Drift.
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.