Across The Dead Earth is a commercial product, but for now, the company offers a free pdf version.
Rules: Post Apocalypse
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.
Savage Survive Skirmish Rules
Savage Survive Skirmish is a set of free wargames rules for games set in a post apocalyptic winter.