D&D retroclones:
SpoilerAdventurer Conqueror King
Basic Fantasy (free)
Dark Dungeons (free)
Dungeon Crawl Classics
Labyrinth Lord (free)
Lamentations of the Flame Princess (free)
Mazes & Minotaurs (free)
Myth & Magic (free)
OSRIC (free)
Swords & Wizardry (free)