GDW’s Classic RPG Traveller from the late 1970s is currently available for free at Drive Through RPG.
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.
Force 9 is science fiction role playing game inspired by Alien and Starship Troopers.
In this unusual free role playing game, you play a simple robot charged with defending the last human being. The Last Human is written by Nathan Russel as a part of the 24 Hour RPG Challenge.
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.