Havent been here for a long time but remembered the aqua-soft forums now while playing around with my iRiver mp3 Player. So if you wanna tweak, mod or extend your current player take a look at the rockbox project. So far i can say it's one of the best open source projects ever for any mp3 player. In comparison to the actual firmwares of most players it can cope with every thing, mostly it makes it much better. Supports lots of codecs, has album art, Games like Doom, Pacman, Jewels, Tetris (all free!!), Translator, Recording support, and much much more...
The only thing which is under development is the video playback, but that will come!
Cheerio Angry
supports the following mp3 players:
Archos Player/Studio
Archos Recorder v1
Archos Recorder 8MB
Archos FM Recorder
Archos Recorder v2
Archos Ondio FM
Archos Ondio SP
iriver H100/115
iriver H120/140
iriver H320/340
iPod color/Photo
iPod Nano 1st gen
iPod 4th gen Grayscale
iPod Video
iPod 3rd gen
iPod Mini 2nd gen
iPod Mini 1st gen
iriver H10 20GB
iriver H10 5GB
iAudio X5
Example iClix Theme for iPod

Example myPod Hybrid for the iPod:

EDIT LINK CORRECTED NOW
Aqua-Soft Forums