New Trent Easypak 7000mAh Portable Triple USB Port External Battery Charger/Power Pack for Smartphones, Tablets and more (w/built-in USB cable)
I bought two of these battery packs for my wife and I when we travel for our Iphones and our kids hand held games.  These replace two I bought about 5 years ago that I wore out.  They work great and are thin enough to fit in your pocket if needed.  I Love New Trent Battery Packs and would recommend them to anyone.