Most computers are set up by default to turn off when the power button is pressed and (of course) to turn on when the power button is pressed again. Knowing this, I simply went into the computer case and saw which two wires are used for the computer power button. These two wires will be connected to a small molex connector which is then placed onto some pins on the motherboard. Simply extend these wires to a pushbutton and you then have a button just like the power button on your computer. Some people splice the two wires together and solder them all together and others simply extend some small thin wire from the molex pin connector on the board. The actual molex connector which is used for the power button on the computer will hold it in place once it is pushed onto the pins on the motherboard. Basically, one pushbutton can be used to turn on and turn off the computer.
As for powering other stuff, I simply purchased some 6 outlet power strips. I cut the thick wire in half and connected it to a light switch. This way, I can always have the power on going to the 6 outlet power strip, and I can control whether my lights, monitors, etc.. are on but flipping the light switch to on/off.
You can see some of the photos of the light switch wiring from my site (pics 50, 51, 52, 53, 54). Click on the link below and select the "Photos" link on the bottom.
http://unclet.arcadecontrols.com/DadsJukebox/DadsJukeboxMain.htmlHope this helps.