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.
Role Playing Game
Icar Science Fiction Role Playing Game
Icar is a science fiction RPG with an intriguing setting:
Icar is a space opera set in the 93rd Millenium. Since leaving Earth, the human race has been through a number of social catastrophes, including the cliche of creating a humanoid race of robots which fought back against their creators. The Imperium, a benevolent organisation, steers the human race to keep it alive.
Take the part of a Scavenger crew exploring deep space for the chance of finding expensive salvage, step into the shoes of a Special Forces team up against the odds, fight crime through the eyes of a policeman as part of the most high-tech police force ever know or just create your own characters to explore this detail rich universe.
The author has been working on this since 1990 and the game is now in its umpteenth revision.
Dead Simple Modern Espionage Rules
Dead Simple Modern Espionage is a set of free wargames rules for modern spy games.
Hulks and Horrors Science Fiction RPG
Hulks and Horrors was an unsuccessful Kickstarter effort, but now is available as a free pdf.
Dungeon Quest RPG
Dungeon Quest is a free old school fantasy RPG.