Broadband Adapter
The GameCube Broadband Adapter (sometimes abbreviated as BBA) is a device that plugs into the serial port 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
There is a kernel driver for it, but since the GameCube port of Wii-Linux hasn't been confirmed working on a GameCube yet it's functionality status is unknown.