Rockbox

Rockbox' is a replacement for the standard firmware in various forms of digital audio players (DAPs). It offers an alternative to the player's operating system, in many cases without removing the original firmware, which provides a plug-in architecture for adding various enhancements and functions. Enhancements include personal digital assistant (PDA) functions, applications, utilities, and games. Rockbox can also retrofit video playback functions on players first released in mid-2000. Rockbox includes a voice-driven user-interface suitable for operation by blind and visually impaired users.

Rockbox runs on a wide variety of devices with very different hardware abilities: from early Archos players with 1-bit character cell-based displays, to modern players with high resolution color displays, digital optical audio hardware and advanced recording abilities.

Rockbox is free software released under the GNU General Public License.

(Source: en.wikipedia.com)

Links
http://www.rockbox.org