Mine runs OS-X Server and does double duty as my iCal + CardDav server and also runs Plex Media Server (
http://www.plexapp.com/ )to manage all my digital content (tv shows, movies, music). It does direct HDMI out to my TV and also streams all that content to Macbooks, iPads and even PCs all over the house with an amazing interface. Torrents run 24/7 on it as well.
They like RAM so load it up with 8 GB and you'll be amazed at how well stuff runs on it just with a 2.66 C2D.