Dead Simple Steampunk is a set of one-page RPG rules for settings in a Victorian science fiction setting. There’s also a one-page setting and one page for equipment.
Searchers of the Unknown is a set of free role playing rules. These are one-page, minimalist rules, each of which is designed as a stand-alone product. The collection includes: Searchers of the Unkown, Dwarven Glory (and supplement), Karry on Krawling, Witches of N’kai, Adventurers!, Cyborg Samurai Are Go!, Dirty Rotten Scoundrels, Funkin Fists of Fu, Jung Guns, Raiders of the Unknown, The Bastards, Brickmasters of the Unknown, Mutant Got Gunz, Mutant Scavengers of the Ruined Earth (and supplement), Tempora Mutantur (and supplements), Searchers in Spaaaace!!!, Scavengers & Spacewrecks, Swashbucklers of Mars (and a supplement), and Song of the Symbi.
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 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!
Microlite 20 is a set of minimalist RPG rules designed for compatibility with Old School Dungeons and Dragons materials, as well as the d20 OGL. Fourteen free Microlite 20 variants are available at the link.