ITPS + Fan
Sure, they may be crummy for your main power supply but they can provide autostart and shutdown control or provide regulation for other devices. Don't want to supply your screen from your PSU? Use the ITPS!
This one was working when I pulled it out of the car. I'll throw in a fan I used to keep it cool. Fan has a thermister that makes it run faster when sensor detects heat. Fan is soldered to ITPS output and runs when ITPS is on, shuts off when ITPS shuts down.
In addition, there are two wire leads used to connect an SPST pushbutton that allows you to start the ITPS without turning on the ignition. This is useful when you take the computer assembly indoors and hook it up to your DC power supply in the house.
$25, shipped in the U.S.