Use it for extra storage (backups)This is what I've done with my old one.It's hooked up for networking, so if I've lost something on my newer computer,or, if I delete and do a fresh install all I do is transfer the files back from the old one.
As DieSse said, use it for Linux. NX needs a fraction of what comedy operating systems like Vista require to support their bloat. Perhaps just use it for simple distribution. An interesting project would be a Linux Live CD Router - click here . 15 gigs of dedicated MP3 server would probably be the most simple M$ thing.