On last weeks episode of Boing Boing Gadgets we discussed the Anker Astro Mini, a powerful, pocket sized, external cellphone rescue battery; Last night a friend loaned me her 6000mAh Anker Astro. At about twice the size of the mini, it holds twice the juice.
The Astro is essentially just a bigger version of the mini, with a weird shake to turn on and off switch (reminds me a lot of the Pax by Ploom.) This unit isn't quite as pocket sized, but it certainly fits in a travel bag and should give you 3-4 good charges to a cellphone. It is simply a battery with a USB port.