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.
Role Playing Game
Doom: The Next Chapter Role Playing Game
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.
World War II Role Playing Game
The World War II RPG by Mikko Kauppinen is designed as a simple game that is deadly to low level characters, but that allows more powerful characters to do very cinematic actions. Characters have three attributes and a number of skills. Action resolution is done by rolling 1d6, subtracting the result from the relevant attribute and adding one. If the result is more than zero, the character has succeeded, and bigger numbers mean greater successes.
Open Adventure Old School Adventure Game
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 RPG
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.