Originally a Commodore 64 expansion cartridge developed by Datel. Other platforms include NES, SNES, Genesis, Playstation, Playstation 2, GameBoy Advance, and Nintendo DS.
There is another variation known as the Pro Action Replay, with extended functionality over the original.
Detailed Pages
Game_Enhancers:Pro_Action_Replay_(SNES)
Game_Enhancers:Action_Replay_(Genesis) - Also contains some information about the Pro CDX device.