Kill a Watt meter
A device which can be used to determine how much electricity an appliance is draining from the wall. It can also be used to inspect aspects the behavior of electricity.
It is easy to mistake the VA setting for the Watt setting, because the indicators for them are very close and the measurements tend to be very similar.
Other brands of the same type of tool include the Seasonic Power Angel.
Power supply tester
A device which turns on a power supply without a computer having to be attached to it and checks whether the voltages are within specifications. It does not put a considerable load on the rails, so its ability to predict how a PSU will work under real world conditions is limited.
Only usable on standard ATX power supplies; some power supplies use a different wire configuration but nevertheless use the same physical connector as standard ATX power supplies. Such non standard PSUs can be identified by comparing the colors of the wires to the corresponding ones on standard ATX models.
A device which can test properties of small electrical circuits, some of its functions are like some of those of a Kill a Watt, but usable for the insides of a computer instead of a wall circuit. It can be used to supplement a barebones low functionality power supply tester which does not display actual voltage readings.