As you know, I spent a long weekend kicking around Los Angeles. My cell phone was doing quadruple duty all weekend; GPS, camera, social networking, and phone. It should have been a challenge keeping it fully charged, but it wasn't, thanks to this little guy on the left:
I carried an external backup battery with me. I'd intended to buy this battery, but when I mentioned it to a coworker, he leant me his, the Trent Mobile Power Pack. The battery worked like a charm; when my phone hit 25%, I hooked up the cables and it got to work. Good thing, because I managed to drive in the completely the wrong direction multiple times. I might have ended up in San Diego without a fully charged GPS.
The battery was roughly the same size as my cell phone, and because I carry a giant bag anyway, it wasn't an added burden. At night, I charged up both the phone and the battery (the battery does take all night to reach a full charge).
It was nice not to have to hunt for coffee shops with outlet access.
Highly recommended; I will definitely purchase one before our next vacation. Get out of the coffee shop and on with your vacation!