I made a MP3 player once using old pc parts
Any pentium above a P75 will do (preferably below P133, they can be run with only a heatsink, reducing the noise) , couple of megs of ram and a hard drive to store the mp3'z on
Also, a sound card (obviously), and an old keyboard (to take apart and use the controller chip)...
Put this all in a small case, put some buttons on the front (play, pause...) using the keyboard controller, an hd44780 LCD driven from the parallel port and write some software for it and you've got yourself an standalone MP3 player to connect to your stereo set.
you could also use an old cdrom drive to play mp3 directly from cd, or a netwerk connection to upload music...
Search the net a bit, lots of ppl done similar project.