Killzone Survival is a set of fan created rules for skirmishes in the Warhammer 40K Universe.
Miniatures Rules
Give Them Cold Steel Civil War Rules
Give Them Cold Steel is a set of free wargames rules for the American Civil War.
Struggle Against Everything and Everbody Mexican Revolution Rules
Struggle Against Everything and Everybody is a set of free wargames rules for the Mexican Revolution of 1910 – 1920. It’s a period I’ve long wanted to game.
Mutant Future RPG
Mutant Future is a retro-clone of the old TSR Gamma World Game. There’s a commercially available printed version, but you can also download an artless version if you just want the rules.