The P3 P4400 Kill A Watt is an electricity usage monitor that works with any device that plugs into AC power. It has an LCD screen that shows the power consumption by kilowatt-hour, and also calculates electricity expenses by the day, week, month, or year. It displays volts, amps, and wattage within 0.2 percent accuracy.
This is particularly useful if you're trying to find something in the house that's consuming a large amount of power. The Kill-A-Watt plugs into an AC outlet, and has an outlet on the front that other things can be plugged into.