Gemini is a set of alternate skirmish rules for Warhammer 40K that use an Activation Roll, Action Point system.
There seem to be quite a few people who love the Warhammer 40K Universe, but who are less than enamored with the rules.
Rules: Science Fiction
Terminus Gate Basic Rules
Spider Monkey Games offers the basic rules for their Terminus Gate game for free. Terminus Gate is a skirmish game set in a distant future.
The site has both printer-friendly and digital friendly versions.
Space Battles
Space Battles is a set of free wargame rules designed for HO Scale science fiction miniatures.
Sulphur Post-Apocalyptic Battle
Sulphur is a set of free wargames rules for post-apocalyptic skirmishes. There’s an accompanying 28mm line of figures.
Killzone Survival Warhammer 40 Skirmish Rules
Killzone Survival is a set of fan created rules for skirmishes in the Warhammer 40K Universe.