Rockbox for Iriver, Ipod, Archos and Iaudio players
Rockbox is an awesome software replacement for the players listed above. It allows dual boot between the original OS and the Rockbox OS.
Since I have gone to the rockbox software I have not used the original OS on my iRiver.
The software is very stable for archos and iriver currently. I have loaded it on my friends iPod 4th gen 20gig, but it needs about another month or 2 of development. However it is great if you want to experiment with it.
I would say the main reason to go rockbox are the following:
It is the only portable audio player capable of true gapless playback. Gapless playback allows you to listen to live music or songs that flow into each other with out the gap of silence in between them,
most versions of the os that have been in develpment for a long time gets better battery life than the original OS.
Supports almost all formats. including FLAC, OGG, AC3, mp3, AAC, etc. Now you can use your player for lossless playback on your stereo. My iRiver also has optical out so no more cd player needed to enjoy hours of high quality music.
No need for itunes or windows media player. Shows up as a removable hard drive and just add your music via folders. Also has a tag cache system that allows to search and play music via artist, genre, etc.
Has tons of games that can be played while listening to music.
http://www.rockbox.org