I have this one: http://www.amazon.com/gp/product/B00H9BECAC/ref=oh_aui_detailpage_o07_s00?ie=UTF8&psc=1
25$, 10000MAH. 2 output ports. One for high output batteries (Razr Maxx, tablets etc.) and one for other batteries (iPhone etc.).
Last week I switched to a waterproof Samsung S5 phone which lets you swap batteries. I bought a couple extra batteries to throw in the pack. I might pick up one of the small D battery size chargers like Grouse pointed out.