I seem to have to post this link quite alot, so I've decided to give it its own thread.

If you wish to replace or modify a power supply for or from a PowerMac G3 or G4, look at this site:

http://www.outofspec.com/frankenmac/wire.shtml

You will have to scroll down a bit, and then you will find a handy comparison showing the pinouts from an ATX PC PSU, a ZIF/Sawtooth (these two are the same) PSU, a DA/Gigabit (also the same) PSU, a Quicksilver PSU and an MDD PSU.

For those with G3s: The Blue & White uses a PSU exactly the same as an ATX, but with the (usually white) -5V wire disconnected.
The original Beige PowerMac G3s (Tower, Desktop and All in One) use bog standard ATX supplies. If you have something older, or newer (G5), I'm afraid you'll have to llok elsewhere.......