Not really retro, just old, but I bought an old rackmount server cheap, which arrived today -
An Intel Server System R1304BTSSFAN, with Intel Server Board S1200BTS. Socket LGA1155, Intel C202 Chipset.
Installed CPU is Intel Xeon E3-1220, Quad Core 3.40GHz. Currently has 4GB DDR3 RAM installed.
Was surprised to see it has a PCIe x16 slot, not that it's particularly useful inside this case.
No idea what I'm going to use it for at present, possibly a secondary room PC for now. I quite like how easy it is to fit things inside, so quite possibly it could become a new server in the future,