Of all of the gadgets I have, I think my favorite is the P3 Kill A Watt. It's a simple, inexpensive power meter (less than US$30.) You plug it in the wall and plug the device you want to measure into the Kill-A-Watt.

I used it to discover that my 15" CRT monitor consumed over 100 watts, while the new 19" LCD consumes only 30 watts. I used it at Christmas to decide how often I want the extra lights on. (It's not hard to burn 1000 watts with Christmas lights!) I found out the VCR, stereo, and DVD player consume enough power to justify turning them off. The main computer is always useful, so I leave it on even though it consumes 75 watts when idle. (110+ when the processor is busy.)

Whether or not it saves me money, I think it's cool to know stuff like that. ;-)