Yesterday, I heard a story about an Uber pedicab driver here at SXSW who was making a lot in tips by pedaling people around and letting them charge their devices via USB battery. Conferences like SXSW chew up mobile device batteries because people are Tweeting and using apps all day long, without any real opportunity to charge up during the day. One of our Lytro team members was so desperate that she climbed over the counter at a bar to charge her iPhone up. I've been going to a lot of conferences lately, and finally bought a [Mophie Juice Pack Plus](http://www.mophie.com/iPhone-4-s/47.htm) because it's more convenient than using an external battery and cable.
It's awesome. It will more than charge up an iPhone from empty, which means that you can share it with your friends as they need power. Also, it is strangely satisfying to hold a larger mobile device. I actually find the iPhone (4S) more comfortable to use with the Mophie case than without. Highly recommended. The Juice Pack Air is even smaller and cheaper, but has a battery with 25% less charge capacity ([people say](http://forums.macrumors.com/showthread.php?t=1194000) that the Air won't quite charge an iPhone from empty to full).