Exosolar is a free rpg
about robots struggling to survive on an alien planet. Players will have to make the most of their limited resources as they overcome obstacles that threaten their mission. This is a ruleslight RPG with a focus on narrative. One player is the GM (game master) and controls the world while the other players are the robots.
Alienstar is a free science fiction role playing game. It is generic and designed to let you play out your favorite science fiction movie settings.
Doom: The Next Chapter is both an RPG, and a tabletop strategy game based in the Doom universe. It is a great example of how role playing mechanics can carry over to miniatures gaming.
Open Adventure is a free, open-source old school role playing game. From the description:
Open Adventure is a full-featured tabletop adventure game that offers out-of-box support for medieval fantasy and science-fiction expeditions. In Open Adventure, you play as a heroic or villainous figure free to explore the perilous world created by the all-mighty Game Master. Along your adventure you’ll battle ferocious enemies, solve complex puzzles, navigate labyrinthine mazes–and much more!
Cybercity Blues is a free RPG that has a lighter take on the cyberpunk genre. The authors write:
“Cyberblues is a bit like Cyberpunk, only more mellow.”
Cyberblues City is a near-future, action-adventure roleplaying game. Fast-playing and often funny, it offers a lighter take on the cyberpunk genre, both in terms of system and tone. Cyberblues City uses a much modified version of the Fudge roleplaying system designed to make it easy on GMs. And best of all, it is absolutely free.