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/15120603