Earth Reborn is a set of free wargames rules for post apocalypse games.
Rules: Post Apocalypse
Across The Dead Earth Post Apocalyptic Skirmish Rules
Across The Dead Earth is a commercial product, but for now, the company offers a free pdf version.
Fallout Skirmish Game
Fallout Skirmish is a set of free wargames rules for skirmishes in the Fallout Computer Game universe. The rules are heavily influenced by Necromunda.