Learn Linux by throwing a copy of Ubuntu or Mint on there?
Add a large HDD and install FreeNAS, and you've got yourself some network storage (if you're ok with the power usage)!
Make a MAME-powered arcade cabinet?
Make a retro gaming box (maybe a little too fast for a lot of DOS games)?
Donate it to a person or an organization in need?
Thanks for the suggestions. I wrote them down. I think setting up a web server is pretty neat idea. It will be a useful experience. I'll look into it when I finish up my current project.
Donate them, or I use them for file backup servers. Instant, hassle-free backups, really gives you no excuse not to have them. FTP is easiest, otherwise if you can't open ports on your router or don't have a static IP, use something like Bittorrent Sync or GoodSync.
A webserver is a good idea too. Keep in mind that for both of these ideas, it might be more efficient to get a low power solution like an Intel NUC or Raspberry Pi (what I recently started using).