Trenched is a set of free wargames rules for squad level combat. In spite of the name, it is not a WWI rules set. The example photos are of Warzone science fiction figures.
Miniatures Rules
American Civil War Riverine Actions
Mike Callan offers this set of free wargames rules for American Civil War Ironclads. The set originally was published in Wargames Illustrated 14.
Politics By Other Means
Politics By Other Means is a miniatures matrix game designed to cover all eras.
Shock Force Starship Troopers
Shock Force Starship Troopers modifies the Shock Force rules by Demonblade for the Starship Troopers film universe.
Trireme Ancient Naval Rules
Trireme, from Andy Watkins, is a set of free naval wargames rules. These are of the one-page variety, which means that there is much to recommend them. I also like the staggered movement system based on boat speed, which reminds me of the old Car Wars system.