I bought the mophie iPhone 3G Juice Pack
. This is a battery pack for the iPhone that connects to the iPhone like a case. I chose this battery pack instead of others, as it would fit in my pocket while it is attached.
The Juice Pack is 1800 mAh battery, that doubles the battery life of the iPhone. It connects to the iPod dock connector. The Juice Pack has a USB port that it uses to chare the battery. If a iPhone is also attached, the iPhone's battery will be charged before the Juice Pack's. Also on discharge, the Juice Pack's battery is discharged before the iPhone's internal, so you can remove it from the battery pack once the battery pack loses power.
The only problem that I have with the mophie Juice Pack is that after I put on a BodyGuardz protector on my iPhone, it takes a significant amount of force to put on and take off the battery pack.