Could this signal be of any use when converting a PC powersupply to desktop usage? Realistically, the answer is YES. A voltage on the PWR_OK line indicates that the PS has completed a successful POST and that the output voltages are stable and within design specification. If you wanted to use an LED (light emitting diode) as an indicator that the PS is on, rather than tie it to one of the +5V or +12V lines, attach the PWR_OK line to the anode (+) side of the LED and place a 220 ohm resistor on the cathode (-) leg before grounding it. The cathode leg is normally shorter on new LEDs -- if the legs have been clipped, the cathode will be on the same side as a flat spot on the LED base. The LED should be bright for normal operation -- it could possibly glow faintly if the PS has withdrawn the signal simply due to bleedover.