Tweet-a-Watt
Tweet-a-Watt is an Open Hardware hack of the commercial product Kill-a-Watt which allows you to measure, store, graph and Tweet your power consumption.
The device is capable of measuring 1800W and is very useful to quantify your power consumption and modify your behaviors.
This is sold as a hobby kit and requires some advanced assembly; all the resources and support you need can be found on the very friendly adafruit.com site.
Photo Credit: ladyada (flickr)