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.
Dead Simple Steampunk is a set of one-page RPG rules for settings in a Victorian science fiction setting. There’s also a one-page setting and one page for equipment.
Living Dead is a set of free roleplaying rules for the zombie apocalypse.
Dragon Slayers is a playest version of an upcoming free role playing game. The author writes:
Dragon Slayers RPG looks like Savage Worlds but plays like D&D 5e. The goal is to have D&D-style adventuring without the fiddly bits. GMing is easy and the game is great for new players and one-shots. It features step-die mechanics and has inherited a few dirty hippie quirks (abstract equipment, 13th Age style backgrounds). Magic is low-ish powered and mages cast with spell points.
Contagion is a role playing game of modern horror, in which the forces of Heaven and Hell are fighting for the souls of mankind.