I discovered Rockbox today. It's a complete open
source replacment for the firmware of several portable audio players
include the iPod. It supports many audio formats including MP3, Ogg
Vorbis and WMA and has cool stuff like gapless playback (great for mix
CDs), themes, bookmarks and much more. I also love the fact that no
special directory layout is required. Use your normal shell commands
rsync) or file management interface (eg. Nautilus) to
copy tracks to your iPod.
I installed it on my iPod this afternoon; wow. It's not perfect (the tag cache system is fiddily) but it's stable and works as advertised. My main criticism is that there's too many options and features to fiddle with; great for geeks but not for the general consumer. But hey, it's open source so someone could create a friendlier version at some point. I love the idea that I can play with the code if I want to. It also has a plugin system for add-on apps which I'd love to check out at some point.
Rockbox is definitely staying on my iPod (yes, the process is reversible).