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.
Rules: Science Fiction
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.
Starship Marines Boarding Actions Miniatures Rules
Starship Marines is a set of free wargames rules for Starship boarding actions. This is quite a detailed set, and is continued in Starship Soldier.