I'm working on a benchtop power supply hack which converts an old ATX power supply into a bench power supply that provides -12V, 3.3V, 5V, 7V, 10V 12V, 17V and 24V. The supply I'm using for this project doesn't have the white wire which would provide -5V and another host of flexibility, but this is a good start. Wikipedia has an article on computer power supplies including a wiring diagram that is very helpful.

Steps,