Steve Jackson’s GURPS (Generic Universal Role Playing System) is one of the grand old men of the role playing world. The game provides one of the most realistic, detailed gaming systems ever. Because the combat system is so good, I know of many miniature gamers who use it for skirmish games. Steve Jackson games has released a free “lite” version of the rules as a pdf download.
Tombstone is a free RPG set in the wild west. It’s designed to be rules-light, and has quick play character creation to get you started.
Living Dead is a set of free roleplaying rules for the zombie apocalypse.
Contagion is a role playing game of modern horror, in which the forces of Heaven and Hell are fighting for the souls of mankind.
Force 9 is science fiction role playing game inspired by Alien and Starship Troopers.