Peripheral device developed by Codemasters and released by Galoob Toys which allows the user to effectively hack video games for a variety of effects, from such practical things as giving yourself infinite lives or ammo or making you immune to damage to stranger things like turning Mario into an indistinguishable purple thing that swims through air. This is accomplished by entering a series of six-to-eight digit codes onto the startup screen, which will modify the game's data or programming.
The original Game Genie was for the Nintendo Entertainment System and allowed you to enter up to three codes (sometimes jokingly referred to as three "wishes", hence the name "Game Genie"). The peripheral proved popular despite Nintendo's disapproval (lawsuits halted the sales of the SNES version on the grounds that they were piracy tools, and they even built a feature into the top-loading NES that prevented Game Genies from working) and versions of the Game Genie appeared for the Game Boy, Game Gear, Sega Genesis, and Super Nintendo (and possibly some other systems). Some of these allowed five "wishes" instead of three.
Game Genies usually came packaged with a book containing codes, and users could subscribe and receive regular updates. In addition, more inquiring gamers would attempt to find their own codes. These days, there are websites where users log and attempt to create Game Genie codes. Most console emulators feature an emulated Game Genie as a built-in option.
The Game Genie spawned one competitor, the Pro Action Replay, which worked in a similar fashion. Both peripherals were retired at the end of the 16-Bit generation. The equivalent for the 32/64-bit era was the Interact GameShark. The next generation of Console Wars, dominated by the PlayStation 2 and Xbox, benefited from a new generation of the Action Replay, as well as the Pelican Accessories Code Breaker. No off-the-shelf tool permits similar enhancement to the Wii, PS3, or Xbox 360 at this time, and with the upswing in online console gaming and remote hardware verification, they may not ever be back. Much of this has to do with newer consoles using modern hardware which can restrict programs from accessing one another's memory, a capability which if misused could result in all manner of cheating tools for online multiplayer games. However, someone did buy the Game Genie name, to use for a save game editor program, and of course this is still very possible in PC games, through software like Cheat Engine.
As such, the Game Genie and related devices have become a footnote in the history of gaming, remembered mostly by nostalgic gamers as a life-saver in the days of Nintendo Hard.