From Bulbapedia, the community-driven Pokémon encyclopedia.
The Transfer Pak (Japanese: 64GBパック) is a peripheral for the Nintendo 64 that allows for the transfer of data between the console and Game Boy or Game Boy Color cartridges. The device came as a pack-in with the first Pokémon Stadium. With it players are able to upload Pokémon and items from their Game Boy games for storage, or to be used in battles and mini-games. It also allows the player to trade Pokémon (two transfer paks required) and even play their Game Boy Pokémon games on TV using the Stadium games GB Tower feature.
Games with Transfer Pak support