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.
Miniatures Rules
Nelson’s Wars Napoleonic Naval Rules
Nelson’s Wars is a set of free wargames rules for naval games in the age of Napoleon.
Imperial Guards Vs Zombies Rules
Imperial Guard vs Zombies is a set of modifications for Warhammer 40K
1450 – 1550 Renaissance Wargames Rules
1450 – 1550 is a set of free wargames rules for playing miniature games set in the renaissance. The author says it’s “slightly more complicated than Tactica.”
Doom: The Next Chapter Role Playing Game
Doom: The Next Chapter is both an RPG, and a tabletop strategy game based in the Doom universe. It is a great example of how role playing mechanics can carry over to miniatures gaming.