Use it for some kind of server inside your house. Store movies, pictures, whatever. Connect all your printers to it and make it a print server. Maybe buy another 30 GB drive and use RAID1 (basically will create 2 duplicate disks to prevent failure - if one drive fails, the other will still work) for your important files. Make it a torrent/download server that you use exclusively for downloading things. You could even make a website (but if you want a domain name, you'll need to pay a few bucks a month)
There's a million possibilities. Try putting Linux on it and experiment with that. Have sweet CounterStrike and DOOM LAN parties!!
This is kinda useless:
Hope this gives you some ideas. I currently have a Pentium II 266 MHz Win98 box set up inside a Legos case that I built as a music server so anyone can wirelessly connect to my network and get access to a few gigs of music. Actually, the uber-geek lego PC wasn't even that hard to set up. I just ripped everything out of the case and put it inside a lego box. Pretty neat.