The Kill-o-watt lets you monitor exactly how much energy you and your appliances and gadgets are pulling down from the grid. It's hard to get a picture for conservation in your house until you actually know what and where is pulling the most energy. It's cheap and easy to use and if you use it right you can save a bunch of money off your power bill.

Plug this unit into the wall. Plug your appliance into this unit. Instantly know how much elecricity the appliance is using (and calculate how much it is costing you to run).

this is my i am a real adult purchase, plug the appliance into the meter then the meter into the wall and watch your money fly out the window, i found that my refrigerator was the queen watt gulper and when i had it serviced it was a little tiny bit of ice that formed on some tube, thus making it run like all the time, my light bill went down 31.00 this month.
this is so worth buying,

This piece of equipment is awesome, not only is it pratical but I find it fun. I can use it everywhere in my house to look for this things that are using energy when I am not using them. I simply love that fact that it allows me to cut down on those things that up my power bill. It also makes a nice measuring device when checking how many Watts or Amps my computer is using. Its important when planning out power requirments for a Lan Party. (via ThinkGeek)

This gadget monitors your appliances' electric usage so you can reduce your bill, and the price is right - only $39.95. (via the-gadgeteer.com)

You can't make smart choices about your energy and environmental future without knowing what you're doing now. The Kill-A-Watt is an inexpensive tool that details the energy consumption of electrical devices -- especially helpful for finding the "vampire loads" coming from stuff you thought you'd turned off.

Keep tabs on how much energy you use (via Gaiam.com)