Broadband Adapter
The GameCube Broadband Adapter (sometimes abbreviated as BBA) is a device that plugs into the serial port (the EXI bus) on the bottom of a GameCube, and exposes an RJ-45 (commonly referred to as Ethernet) plug. It's intended use is for online or local multiplayer games.
Compatibility
The BBA has a driver in Wii-Linux, but since the GameCube port hasn't been confirmed working, the functionality of the driver is unknown.