Don't be so smug the next time you walk away from a trade show with a mountain of free flash drives. They require electricity too, and with this simple USB power monitor you can calculate just how much those supposedly 'free' giveaways are contributing to your monthly power bill.

The monitor sits between a USB port and a given device, and reports the voltage and current being used in either direction. It's a great way to calculate just how big a backup battery you might need to keep your phone running, or whether or not you should actually leave that novelty wriggling USB tentacle plugged in all day long. [Smartronix via The Gadgeteer]