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,

1. Short the green wire (Power On) to black or install a switch on it. This will tell the PSU to turn on.
2. Group the black, orange, yellow and red wires according to color.
3. Put these each into some configuration that you can connect to them. Binding Posts are a popular option I expect to use. My first run has screws and washers going through the hole of a ring terminal that the wires were crimped into.
4. Connect something that needs power and have fun!