Knowledge is power — but it won't charge your gadgets. While you're zipping around campus all day, ensure that your tablet, ereader, smartphone, or MP3 player has enough juice to last with iGo's Power Trip 4700 ($60). The backup battery, with its slim, lightweight profile, is very portable. Plug your device's cable into the USB port, and you're good to go. An easy-to-use push button tells you how much charge is left in the Power Trip.