RockBox is a set of "open firmware" for various MP3 players, including the iPod, Archos and iRiver players. The firmware is the operating system for the player, which determines how the player behaves, what programs it can run, and what restrictions it puts on your actions. By loading an open firmware on your player, you get total freedom and lots of bonus features, like video-games and other third-party software. The code itself is free/open source software and has a giant and active developer community that's always adding new features to it.
(Thanks, Pat!)
