Cdred is a Linux driver and two programs that enables *any* CD player software (xfreecd,gtcd,etc.) to use audio cdroms inserted in other computer. In other words: you can listen an audio cd in a Linux box that doesn't have that cd inserted. Cdred works in all 2.2.x kernels and support for 2.4 will be implemented soon.
Cdred was written in C and it uses a client-server model to implement most of the cdrom ioctls. The communication is made with RPC. If you want a very clear scheme of the implementation, watch this picture that my sister (7 years old) did. The disc that appears in the right is Meteosat's Espunk. Meteosat is one of my favourite bands and my brother Ignacio is one of its members.