The Jim Dandy/mappinghell server that's right you're browsing it right now!
It's a Compaq EVO with small form factor.
CPU: 2GHz Intel
RAM: 512MB (SDRAM?)
HDD: 2 x 1TB SATA HDDs, on a PCI SATA controller (MoBo does not have native SATA), running in software RAID 1.
OS: Debian Squeeze.
Runs 24/7, in my room. It's pretty silent too.