Kill-a-watt is a nifty, pretty reasonably priced outlet meter.
Plug the Kill-a-watt into a standard 110V US outlet, then plug the device (like a TV, air-conditioner, computer, etc) you want to measure into the Kill-a-watt. The KAW will measure voltage, amps, watts, Hz, kWhr, and elapsed time (since plugged in). The unit requires no batteries (it's plugged into the wall after all) and does a pretty good job. I bought two so I could measure intermediary loads, and of course so I could take one apart and not have to worry about destroying it.
Remember: Power (Watts) = I (current in amps) * V (volts)
To give you an idea of the world I play in, 5V at 1A is a huge amount of power (5 watts)! Most of my projects use under 5V@200mA (1W).
Also realize the cost of a kilowatthour (the amount of 1000 watts used over an hour) varies widely between states and countries. Checkout the average kWh price across the US. For Boulder, CO we're looking at about $0.10 per kilowatthour. Sorry Hawaii ($0.33)!
My UPS uses 5 watts. When a KAW is plugged into wall, and a second KAW is simultaneously plugged into UPS, we can see how much power is actually delivered to the load (my computer) and how much is used up by the constant charging of the UPS battery. Subtracting, we see that the UPS is drawing 5Watts to trickle charge the internal battery (416mA@12VDC is pretty large).
My 19" LCD screen uses 50W, and I have two screens (100W). Thank goodness we have something called 'Power Save Mode' built into most modern monitors. My monitors automatically shut themselves down after a few minutes of non-use.
My computer (a modest 2.4GHz AMD 3700+ with dual HD), uses 270Watts. Not too bad.
The new SparkFun coke machine uses nearly 500W of power, 12kWh per day. It's brand new and the thing never turns off! You'd think it would either reach a cold temperature (my 15 year old fridge turns off!) or power cycles at night (who needs a tasty beverage at 4am?). Ahh well.
My Tivo! Oh Tivo. How you insist on recording Sabado Gigante even when I swear to you, I don't speak Spanish. TiVo is on all the time, there is no power down mode, and thankfully, it only uses about 30Watts. Not too shabby.
My trusty soldering iron uses about 60Watts. It's a bit of a hog. I usually remember to turn it off. Actually, I religiously remember to turn it off. A burned down house is no fun.
Now with everything off (computer, bumping stereo, iron, monitors) my desk uses 24Watts. What is using it? The WiFi router and cable modem. This is something I could turn off. I really don't need to have my computer or internet stuff on while I'm out of the house. How much does it cost me? 365 days * 24 hours/day * 0.024kW/hr * $0.1049/kWh = $21 / year.
Not horrible, but wouldn't it be handy if I had a way to turn that and all the other random junk plugged into my outlets when I left each day?