Pathfinder is Paizo's refinement of Dungeons & Dragons 3.5 edition rules, made after Wizards of the Coast released 4.0 edition and upset a lot of fans because it was completely incompatible with 3.5 and was a sharp change in play-style.
Dreamscarred Press has made a habit of taking Dungeons & Dragons's alternate magic systems and porting them to Pathfinder.
Psionics were revamped in their Ultimate Psionics book
The controversial Tome of Battle was revamped in Path of War
Magic of Incarnum's chakra was reworked in Akashic Mysteries
The OGL fostered many of these for Advanced Dungeons and Dragons First Edition: Among them are Castles and Crusades, OSRIC, and Labyrinth Lord.
Cheapass' Games Kill Doctor Lucky is a spiritual prequel to Clue.