The coolest little box ever: NSLU2. A small, cheap, and silent little thing which runs linux.
Follow the instruction at NSLU2 to mod it, and you’ve got yourself a web-server, ssh-server, file-server etc also capabel of running mdaap (itunes-server).
You could also have a look at Linksys’ own NSLU2-page.
Drawback: Since it’s running Linux, and the default filesystem installed on the disk is EXT3, it’s not so easy to share your disk with friends (who typicaly run Windows).