OSR

Rather than being one game system, OSR is a collective term for games based on the original fantasy roleplaying game. OSR is generally accepted to mean Old School Renaissance, reflecting the use of, but expanding upon the original mechanics of the game.
Broadly speaking, these titles are compatible with many iterations of the game without any conversion work. Even the more modern versions work if the GM is prepared to spend a little time on conversion.
Examples include Castles & Crusades, Swords & Wizardry, Old School Essentials, Dungeon Crawl Classics, and Basic Fantasy (not to be confused with the Basic Role Playing system by Chaosium).