Rockbox
Rockbox is a GPL-compliant open source operating system for portable digital audio players (DAPs). The Rockbox Project began in 2002 and was first implemented on the Archos Studio DAP because of owner frustration with severe limitations in the manufacturer-supplied user interface and device operations.
Rockbox can completely replace the host device's operating system firmware and has matured to become an extensible, flexible platform that provides a plug-in architecture for adding PDA functionality, applications, utilities, and games, and has also managed to retrofit video playback functionality onto DAPs first released in mid-2000. Recently, Rockbox now includes a voice-driven user-interface suitable for operation by blind and visually impaired users.
Although Rockbox's official title is "Rockbox: Open Source Jukebox Firmware", in many instances it is not actually installed to (or run from) flash memory. Instead a minimal bootloader is installed in the supported device's flash which is capable of either loading Rockbox from the hard disk or, alternately, the original factory firmware.
External links
~ Text taken from Wikipedia entry for Rockbox