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.
Miniatures Rules
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.
Mutants and Machine Guns RPG
Mutants and Machine Guns is a free RPG set in a post apocalyptic future. It is simple enough that it fits into the terrific PocketMod format.