Mophie Juice Pack Plus for Iphone

rechargeable external battery case for iPhone 4 and 4S
Gives you up to 8 additional hours of talk time, great extra power for when you can't find/forgot your charger. $60/OBO
Here is a link for more info:

http://www.mophie.com/shop/iphone-4s-4/ ... phone-4s-4