The ITPS is supposed to be a voltage regulator and startup/shutdown controller.
The Carnetix is exactly the same, however, unlike the ITPS, it actually works with the engine off and when "cranking" (starting the engine)
The Opus is a complete ATX power supply, that has regulation in, plus a startup/shutdown controller.
If you don't have a case with a 60W DC-DC power supply, get the Opus. If you do have a case with the power supply in already, get the Carnetix.