Transfer Pak

The Transfer Pack is an accessory for the Nintendo 64. It plugged into the slot of the N64 Controller and had a slot for Game Boy Color games to be inserted. This device allowed for data to be exchanged between the Game Boy and the N64 games. Notable Mario games that used 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 allowed 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