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.
Mythosa is a free fantasy role playing world for any RPG system. It’s a very complete work, and cold be used right out of the box.
Modiphius Entertainment currently is running a Kickstarter for a Conan Role Playing Game. They also are offering a set of free Conan Roleplaying Game quickstart rules on DriveThroughRPG.
Metagaming’s The Fantasy Trip is one of my favorite games of all time. It’s one of the core products that drew me inexorably into the gaming world. Here’s a page with a great number of resources for The Fantasy Trip.