One of the best Old School Revolution fantasy games out there. See more in the Design Room article.
Looking for a digest-sized modular presentation of B/X OSR gaming? My favorite is B/X Essentials, and this one looks to be a solid framework you can use to create many B/X games from as a great base and also reference guide for all the rules you need to get started.
Another classic game, this one plays like a hybrid of the classic fantasy game's basic set combined with the best of the "advanced" books. See more in the Design Room article.
Interested in late-1970's post-apocalyptic science fantasy gaming? This is your game of wild mutants, humanoid animals, and enough ruined exploration and strange societies to keep you gaming for a long-long time past the end of the world.
Other Free Games
Worthy of Mention
FATE (Evil Hat)http://www.evilhat.com/home/fate-core-downloads/
Price: Pay what you want for the PDF. An amazing interpretive, player-driven narrative game that is great for verbal overplaying and storytelling.