Earth Reborn is a set of free wargames rules for post apocalypse games.
Across The Dead Earth is a commercial product, but for now, the company offers a free pdf version.
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 is a set of free wargames rules for games set in a post apocalyptic winter.
Carnage are a set of free wargames rules for auto combat in a slightly different world. The author explains:
Carnage is a racing battle game set in Britain in 2003 or so. Players take control of vehicles from humble Reliant Regals to top-spec Lamborghinis, and then kit them out with improbable Equipment and weaponry, with a view to slaughtering similar minded opponents. They take part in “duels”, (races against each other for honour, pride or cash….but mostly cash), in order to better their machines and skills for the next race. What the end goal is of all this, probably no-one knows, and the Police seem strangely absent from all this. Ah well, who said reality was fun?