These aren’t exactly free wargames rules, but if you already own a set of Warhammer 40K, this is worth checking out. Jervis Johnson has provided this page of colonial amendments to Warhammer 40K. These also would be a good way to introduce fantasy and science fiction wargamers to historical miniatures games.
Warhammer 40K variant
Imperial Guards Vs Zombies Rules
Imperial Guard vs Zombies is a set of modifications for Warhammer 40K
GrimDark Racing One Page Rules
GrimDark Racing is a set of simple rules for races in the Warhammer 40K universe. Just use models you already have. There’s plenty of violence, of course.
Warhammer Napoleonics
Warhammer Napoleonics is a set of stat lines and modifications for using the basic Warhammer Rules for the Napoleonic period.
Killzone Survival Warhammer 40 Skirmish Rules
Killzone Survival is a set of fan created rules for skirmishes in the Warhammer 40K Universe.