I use kill-a-watts and other such tools at work (datacenter). By far the best watt meter is the Belkin Conserve Insight. Easy to find, cheap, goes to 1800w, but the killer feature is that the display part is on a cord, not at plug level like the killawatt. So you can put it on your desk and directly monitor the results of your OC'ing, undervolting, etc.
http://www.walmart.com/ip/Belkin-Conserve-Insight-Energy-Use-Monitor/15120603I've got two of those Belkins, they are awesome. That was how I found out that when I overclocked my Avalon it was pulling 890 watts from the wall with the stock Enermax power supply. I upgraded to a Corsair and now with the same overclock it only draws 730 watts.