I might be on the wrong lines here, but Everest Home Edition provides the (flutuating) voltage readings from the PSU, plus CPU and mobo temperatures, on a real time basis through its Computer>Sensor link.
I do appreciate that a sensor may not be quite as accurate as a true tester, nor will the test be just on the PSU itself.
martjc: not sure where you get your info. from, but in my experience the vast majority of PSU problems are catastrophic rather then intermittent. Stuartli: Everest OK if the PC is working, but what about those occasions when it doesn't, and you suspect a PSU problem? At that price I'm going to buy one next time I visit my local Maplin.