ZAGG makes a nifty road warrior tool that is about the same size as an Apple Macbook power supply that can recharge an iPhone4 about 4 times from absolutely dead. This little wonder battery has 6000 mAh of power, it has flip out wall charger prongs, a 4 LED power meter, two USB ports – one standard power and one higher power for iPads etc. I have been using the 1.0 version, which is slightly less powerful, and it is a regular part of my travel tech backpack. Basically just about anything that needs USB power can be used with it.
My only complaint is on the wall plug, the internal plastic piece that keeps both wall prongs together broke rather easily, it still works but when you fold the power prongs closed now, they are independent and feel loose. I suspect the maid in the hotel pulled it out with some side load and broke them, when I came back to my room it was unplugged and laying on the desk. That part seems to have been designed rather cheaply.