Sulphur is a set of free wargames rules for post-apocalyptic skirmishes. There’s an accompanying 28mm line of figures.
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.
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.
BioSyndrome Zombie Survival Horror Game
BioSyndrome is a community created game of zombie survival horror.
Come With Me If You Want To Live Board Game
Come With Me If You Want To Live is a tongue-in-cheek board/card game in which players try to gather the most followers to save from the apocalypse.