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.
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.”
Shellshock Combat System
The Shellshock Combat System is a comprehensive set of free wargames rules for WWII, Modern and Science Fiction combat. Designed for micro scale (1/285), it features three “levels of play”: basic, advanced and “blitzkrieg” (fast play).