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.
Dropship Horizon has a good idea for turning pine cones into weird looking alien trees.
Hitchhiker’s Quest For The Galaxy is a home-brew card game based on the book of the same name.
You gotta love a game which specifies that “the most depressed player goes first.”
Mutant Future is a retro-clone of the old TSR Gamma World Game. There’s a commercially available printed version, but you can also download an artless version if you just want the rules.
Mutants and Machine Guns is a free RPG set in a post apocalyptic future. It is simple enough that it fits into the terrific PocketMod format.