Atomic Highway is a set of free rules for post apocalyptic role playing.
RPG Science Fiction
The Void Lovecraftian Science Fiction RPG
The Void is a free RPG with a science fiction/ Lovecraft setting.
GURPS Space 1889 Conversion
I loved Space: 1889. The setting was fantastic, and I also liked the RPG system. But for those who prefer a more nuts and bolts operation, you could try this GURPS conversion for Space: 1889
Dice Chucker Universal RPG
Dice Chucker is a rules light RPG.
Dice Chucker is a rules-lite system that puts a new twist on rolling dice! A lot of players like rolling dice and Dice Chucker is here for them. You will find that Dice Chucker is chock full of dicey goodness! The rules here are pretty simple and straight forward and the details are meant to be handled by players and GMs as suits their tastes and the storyline. Dice Chucker’s rules can be adapted to virtually any genre you like: fantasy, sci-fi, horror, western, etc. Let your imagination be your guide!
Dungeon Squad: Adventures in the 41st Millennium
Dungeon Squad: Adventures in the 41st Millennium is a RPG set in the 40K universe.