If you're going to live in a more sustainable way, you have to be able to figure out what you're doing that is unsustainable in order to change it. Tools like ecological footprint tests are good for getting a general sense of your status, but sometimes you need more specific information. Take energy use, for example: how much power do you use? Your monthly electric bill gives you the total, but how can you figure out which of your various toys & appliances need to be replaced with something greener now?
Electric power consumption meters usually cost $80-$150 (according to a spot check at a local electronics superstore near here last week); while you'd eventually make that back from lower power bills, that still feels fairly expensive for non-professionals. I was pleased, therefore, to discover the "Kill A Watt" meter, intended specifically for consumers wanting to figure out how much power they're using at home. The price is definitely much better than professional units; you can find them for under $30. I bought one, and have found it quite easy to use and very informative.
Plug the appliance you want to check into the front, plug the Kill A Watt into the wall, and the system will show you how much power you're using. The kilowatt/hour + time readout makes it easy to figure out your annual draw for appliances, to see how your current units compare with Energy Star models; my refridgerator is now at the top of the "must replace" list, as it pulls nearly twice what the best Energy Star models of equivalent size draw. I've taken to checking pretty much every electric device in the house, out of a mix of diligence and curiosity.
It's a solidly-built device, and although it's really not meant as a pro tool, I can imagine it becoming part of the standard toolkit of greenpunks and sustainability geeks everywhere.