PC SW controller shield for Raspberry Pi
PC SW Controller for Raspberry Pi, very simple and only needs some transisters and resisters. You can also use for general level shifter for 1 input and 1 output.
Connect this board to Raspberry Pi's pin header and install WebioPi.
Connect "OUT" terminal to the power switch in your PC for power control and "IN" terminal to 5V or 12V output of your PC (or USB VBUS) for monitoring PC's Power Status.