Using a Dell D220P-01

In case you don't know what a Dell D220P-01 is - it's a power supply for the SFF version of Dell's Optiplex 740 series PC. The neat thing about it is that unlike a traditional ATX PSU, this one is 12V only, so you get 220W @ 12V (18A) maximum and the PSU should not complain too much.
Another neat thing about them is that the PCs are legitimately junk, so you can get them even for free.

Stolen from PcHub.com

The pinout is in the picture, "P5" has to be connected to ground in order for it to turn on. Unlike newer server PSUs, this one can be connected straight from the start.
If you're like me and don't feel like butchering the connector, you'll need a breakout board. The connector is mechanically (!not electrically!) compatible with an 8-pin EPS12V power connector (plug is Molex 39-01-2080, PCB socket is  Molex 39-28-1083, terminals are Molex 39-00-0168 and Molex 44476-1111).
The 8-pin PCIe power connector is different with the placement of the square and hexagonal holes and is not mechanically compatible...not like that will stop the 200 pound gorilla from trying to jam it in...
Needles to say that if you plug this (and turn it on) into an EPS12V connector of a working motherboard, you will turn it into a non-working one, as the voltage is inconveniently reversed and the power supply will turn on. Results may vary from nothing at all to a spectacular fire and mini explosions of the caps.

"EPS12V"
stolen from http://www.playtool.com/pages/psuconnectors/connectors.html#eps8

If your junkbox doesn't contain a motherboard with an 8-pin EPS connector, fret not, there is an alternative!

24-pin ATX...cut on the red lines...
stolen from wikipedia

Yep, it's not a true hack unless it involves a hacksaw!
side note - on any newer-ish junk motherboard, I suggest cutting the connector out, as the inner power layers use thicker copper (for heatsinking) and carry away heat from the soldering, making it very difficult.

As said earlier, plugging this PSU here to the MB is also not a good idea, although it should not turn on (results may vary). If the PSU does turn on, it will be short-circuited, if it "wins" over the short (burns through), it will send 12V to a 5V line...

Here's my quick and (really) gritty breakout board.

front

back

Yes, it's fugly, yes, it could've been nicer...I was going for "fugly, but functional". The crapton of solder should make sure that it can actually carry the 18A the PSU can deliver.
I kept the +12V rail from being at the edge, as it lowers the chance of an accidental short. The brass washers are actually soldered to the board.