There are many things you could do... there are several FOSS projects that will enable you to turn that box into a full fledged DHCP server/firewall, or a PBX box. Those are the two most interesting options to me, other than hosting a file server.

I just installed Server 2003 on my box tonight so that I can get back into C# Development... if you can get your hands on that (if the system can handle it) you could play around with that?