I own a Sansa MP3 player. A few months ago I learned about RockBox, a replacement firmware for various MP3 players. I tried it and loved it. It’s like putting Linux on a computer. It’s more customizable and you have new features galore.
The other side of the coin is that, like Linux, it’s not without its glitches. For instance, you can’t sync via USB with the Rockbox firmware. When you connect, it fades to a white screen then loads the default firmware to do the syncing. That wasn’t a deal breaker for me, just a slight nuisance. You can play games on it (even Doom!), view actual JPEG images and video files without having to convert them. You can record audio direct to MP3s instead of WAV. It also handles regular playlists (even off the SD card) which was a big help as I didn’t like how the factory firmware dealt with playlists.
I upgraded to the latest version on September 23rd, then learned from Lifehacker that the 3.0 version was released on September 24th. I’m now running the stable version. To upgrade, just copy the new files on top of the existing installation.