Embedded opensource does not need to be unix-like

Today while looking for some embedded Linux Hardware for libera, I stumbled upon Rockbox, which is an alternative GPL firmware for a variety of MP3 players like Ipod, and Iriver. I found this project quite interesting because:

1. they support quite a number of device including mine ( iriver H340, one of the few MP3 players who supported ogg vorbis out of the box )
   
2. they took the approach to write a firmware from scratch, and not try to port Linux which would be probably overkill for these kind of devices ( ARM cpu, or Motorola Coldfire )
   

I am looking forward to having a try, especially they boast a port of Doom !