Transfer Pak

The Transfer Pack is an accessory for the Nintendo 64. It plugs into the slot of the N64 Controller and has a slot for Game Boy and Game Boy Color games to be inserted. This device allows for data to be exchanged between the Game Boy and the N64 games. Notable Mario games that use this device are Mario Golf (Nintendo 64) and Mario Golf (Game Boy Color), and Mario Tennis (Nintendo 64), Mario Tennis (Game Boy Color) and the Mario Artist series (Nintendo 64DD). This transfer pack allowes for specific characters to be unlocked in either game.

Trivia
and the successor of the Super Game Boy.
 * The Transfer Pack is essentially the predecessor of the Nintendo GameCube Game Boy Advance Cable