A battered old Supermicro server from 2009 I picked up for pennies on fleabay. I don't play any modern games or do any heavy number crunching (aside from a few big Excel sheets) so it suits me fine:
733TQ-665 case with PWS-665-PQ PSU and X8DAL-i motherboard
Dual 3.6 GHz Xeon X5687 CPUs
48 GB DDR3-1333 registered ECC RAM
Blu-Ray drive
1 TB SSD + 9 TB of RAID 5 HDD storage
Windows 8.1 Pro
2 GB GeForce GTX 760
USB 3.0 PCIe card
Integrated Realtek audio and Gigabit Ethernet
I also tried to put in a SATA-III card but the BIOS didn't like it. (It only works if I disable the integrated RAID 5 controller, which I don't want to do.)
ETA: A year on, and I'm still using this old thing. These are the current specs:
733TQ-665 case with PWS-665-PQ PSU and X8DAL-i motherboard
Dual 3.6 GHz Xeon X5687 CPUs
48 GB DDR3-1333 registered ECC RAM
Blu-Ray drive
1 TB SSD* + 12 TB of RAID 5 HDD storage
Windows 8.1 Pro*
2 GB GeForce GTX 960
USB 3.0 PCIe card
Dell PERC H700 SAS-2 controller
Integrated Realtek audio and Gigabit Ethernet
* It now also has an Icy Dock MB996SP-6SB with three bays connected to the SAS controller and three to the SATA controller. I actually have a bunch of different operating systems on SSDs and switch between them as needed. Windows XP runs very well on this rig.