OSR, which stands for Old School Renaissance, usually means stuff that is compatible with old school (pre-d20) Dungeons & Dragons. While these older editions have some differences, stuff that is made for one can easily be used for any of them. If you're interested in finding out the differences in the D&D editions, take a look at this geeklist on RPGgeek.com.
So without further ado, the list of free OSR RPGs!
Arcana Rising - a modern-day urban fantasy role-playing game
Basic Fantasy - a clone of B/X D&D with race separated from class and ascending AC. I've reviewed this on the Reviews page.
Dark Dungeons - a retroclone of the D&D Rules Cyclopedia
Delving Deeper - a retroclone of original D&D
Exemplars & Eidolons - a fantasy OSR RPG where you play legendary adventurers
Fantastic Heroes & Witchery - a fantasy OSR RPG
For Gold & Glory - a retroclone of AD&D 2e
Full Metal Plate Mail - a retroclone of original D&D
Godbound (free edition) - a fntasy OSR RPG where you play divine heroes
Hulks & Horrors - a sci-fi OSR RPG
Iron Falcon - a retroclone of original D&D plus the Greyhawk supplement
Labyrinth Lord (no art) - a retroclone of B/X D&D
Labyrinth Lord Advanced Edition Companion (no art) - adds AD&D content to Labyrinth Lord
Microlite 74 - simplified versions of the original D&D rules
Mutant Future (no art) - a Gamma World retroclone using B/X rules
OSRIC - a retroclone of AD&D 1e
Renegade - influenced by original D&D
Shinobi & Samurai - simplified original D&D in an oriental setting
Stars Without Number: Original Free Edition - a sci-fi OSR RPG
Stars Without Number Revised Edition (free version) - a sci-fi OSR RPG
Swords & Wizardry - the Complete version is a retroclone of original D&D with supplements
More to come!