A purple and grey cable with a square block in the center. Can be used to connect multiple Gameboy advances together for multiplayer. The center square block also had a connector so multiple cables could be connected together, supporting up to four players.
http://www1.linkclub.or.jp/~clubey/wolf.gifs/GBA%20graphics/GBA.cable.jpg
http://pocketmedia.ign.com/media/news/image/spaceworld2000/gbacablediagram.jpg