Heh, I've got a DL-320 (I think that's what it called. The 1U version) with dual P3 1.3 GHz, 2GB RAM and 2 x 18 GB 15.000 RPM SCSI drives that I'm not using too. I need a datacenter somewhere to put it up, if I ever find a use for it, as it's simply too noisy to have running at home.
These servers are probably best a web or database servers, as their storage is fast but there isn't much of it. Gameservers are also an option, I have brought a rack server to a LAN party before to run BF 2 gameservers from it :)