Hammer Wars 1.5 is a set of one page wargames rules that mashes up Warhammer 40K and Warhammer Fantasy: What if a 40K troopship crashed on the Warhammer Fantasy World.
Rules: Science Fiction
Antares Free Beta Playtest Rules
Warlord Games has released its upcoming Antares science fiction rules for playtesting. Written by Rick Priestly, the pdf looks very good at this point.
Activation Roll, Action Point Alternate Skirmish Rules For Warhammer 40K
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.
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.