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.
OPUS is a one page, universal role playing system.