Rockbox is a firmware that enhances the functionality of your MP3 player, giving you options the manufacturer's software never had (providing you have a supported player). The 3.0 firmware is the first official release since September 2005 and makes a number of improvements over the previous 2.5 version. Rockbox works on a number of players from Apple, Archos, Cowon, iRiver, Olympus, Sandisk and Toshiba and adds support for a multitude of file formats (such as FLAC). You'll also get games to play (including DOOM on certain models) and generally a whole lot of control over what your hardware can do. You can read the full release notes here.