Fluke VoltAlert voltage detector
"Did you turn on the power switch?" has been a tech support joke for years, but knowing if the power outlet is even on requires the Fluke VoltAlert voltage detector. It uses inductance so that you can run it along a power cord, and it will light up if power is flowing through it. It only works for AC power, but 120V or 220V both function fine. It saved the day when I used it, only to find a missing phase during data center installation.