Use it for a file server, a router, or just to experament with.
Stick a big HD on it, buy a DVD burner for it, put a UPS on it, install linux on it and share files via SAMBA.
With the Linux OS and the UPS for the occasional brown out you can have a nice safe place to store files long term. Expect 300+ day uptimes.
Like movies or music would be great for it.
Then use the DVD burner to burn backups for your most important data. If you install Fedora or Debian or Gentoo on it you can use package managers (Apt/Yum for Fedora, Apt for Debian, Portage for Gentoo.) to easily keep it up to date. Very important.
Then if you want to mess around with it more you can use stuff like Video lan to make it a video streamer, or Ice caster for audio. You can setup a FTP site on it, or do a website from your basement.
Lots of stuff.
To remotely configure this stuff from your windows computers you can use Webmin for a webbased configuration tool, A vnc server to run a GUI(don't need to actually have X windows running at the time), and Putty.exe for a command line thru SSH.
Also there is specific distros created to do this like
Clarkconnect. They also have support and options for professional usage like a office server for a small business, but the home edition is free.
Personally I prefer to build it myself using Debian or another distro, though.