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, however it can be used much like any other Ethernet interface, given appropriate software.
Compatibility
The BBA has a driver in Wii-Linux, and has been confirmed to be fully working.