+1 for Anker
I like to use this one when Im on the go. comes with a built in micro usb charger and is about the size of samsung galaxy phone.
Anker Astro Slim2 4500mAh External Battery
you can also get it in 6000 mAh if you need that extra juice
for something bigger, anything from the LIMEFUEL brand is pretty solid too.
both brands have pretty positive ratings on amazon