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.
science fiction
Mass Effect Tabletop RPG
The Mass Effect RPG is an homage to Bioware’s first person science fiction shooter of the same name.
It’s a well done product, and if it interests you, download it before Bioware’s lawyers get ahold of it.
Warband Science Fantasy Role Playing
Warband is a free RPG with a science fantasy, science fiction flavor. In truth, it could very well be the Warhammer 40K RPG. The author writes:
This game is my homage to the ‘grim, dark future’. A game of spikey power armor wearing, bolt-gun shooting, chain-sword waving chaos worshippers. Characters struggle to survive in the warp of space, as low-level members of a Chaos Lord’s warband, they strive to make a name for themselves and eventually captain a ship of their own.
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.