Give Them Cold Steel is a set of free wargames rules for the American Civil War.
Miniatures Rules
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.
Balkan Wars Rules
The first and second Balkan Wars were fought in 1912 and 1913 between the Balkan League and the Ottoman Empire and later, Bulgaria. Here’s a set of rules for gaming the somewhat obscure Balkan Wars.