Rockbox is an open source firmware for mp3 players, written from scratch. It runs on a wide range of players:

* Apple: 1st through 5.5th generation iPod, iPod Mini and 1st generation iPod Nano
(not the Shuffle, 2nd/3rd/4th gen Nano, Classic or Touch)
* Archos: Jukebox 5000, 6000, Studio, Recorder, FM Recorder, Recorder V2 and Ondio
* Cowon: iAudio X5, X5V, X5L, M5, M5L, M3 and M3L
* iriver: H100, H300 and H10 series
* Olympus: M:Robe 100
* SanDisk: Sansa c200, e200 and e200R series (not the v2 models)