Boost 4200 by Eton ($54.99) is a pocket-sized backup battery pack for your iPhone or iPad.
It measures, 5 inches by 2.5 inches and its slim design allows the Boost to easily fit into a pocket or purse while you're charging your device or just by itself for some power peace of mind.
What I like about the Boost is it's ability to let you know how much juice is left in it. When you pick up the Boost and shake it, it has an LED charge indicator to monitor the battery charge level. That way, you know when you plug your phone in it's going to get charged.