Muse is a long-standing project of mine to address my needs of an MP3 player. It started out as a 386 box which would boot off NFS and allow me to "remote control" xmms on my main computer. These days, it's based in python and runs on a laptop in my car, with an LCD display. I also use it as my desktop player, as it handles some things in a way I like. It's pretty esoteric at the moment, though. 
My second attempt started out as a project written in C/C++. It was implemented on a standalone P166. I had fun spraypainting it black, and I built a little infrared receiver and fitted it inside of one of the ventilation holes. This enables the music software to be controlled via an ordinary remote (I use the remote that came with my amplifier).
The latest attempt is what you can see in the pictures on this page. If you're looking to build an MP3 player for your car, something here might be useful. I have a friend who also uses this code in his car. Here are some photos of what Muse was developed on.
And here's the code.