I made this on the weekend. It's a converted ATX PSU, I cut the cables off and added binding posts to use as a bench-top power supply, I could have simply bought one of those cheap ones on ebay from china for ~£40 however I had this lying about and it cost me about £6 in parts.
First I took apart the PSU and took the PCB out after marking the top heights of the components on the case so I could figure out how to arrange the binding posts & fuses. I then drilled and sanded the holes (time to buy a hand reamer).
The wires colours correspond to the voltages:
Yellow - 12V
Yellow/Black - -12V
Orange - 3.3V
Red - 5V
Black - ground
I desoldered a bunch of these which I didn't need clogging up the case, and then cut the others down to a reasonable length and heat-shrinked them (so far no shorts). I then soldered the leads corresponding to the voltages I wanted; 3.3V, 5V, 12V and ground to their corresponding fuse holders and then from the fuse holder I soldered wire with crimped connectors on the end (to be used with the binding posts), hooked them up and voila, all I had left to do was to attach a switch from ground to the thin green wire which turns the power supply on and off when it is plugged in.
I mounted the fan the other way around as the binding post obstructed it from being in it's original position, I also put the fan guard on underneath as I didn't want the fan and the wires to hit each other. I used 5A quick blow fuses as these things can output ~20A and I'd rather not see my projects go up in flames if I can help it.
If you're looking for a 'how-to' vs. an 'account' I suggest looking at these links, which I used when making this power supply, I see no need to saturate the web with another set of instructions!
http://letsmakerobots.com/node/1355 (I'd like to note I do not use a load resistor, I didn't buy one and the supply seems to function ok without it)
One final note, if you do undertake this project, be very careful of the power caps inside, I left my supply off for a week before touching it, I'd rather be on the safe side!