You probably have this first one already. It's an adapter for charging in your car. Whether you're in your car when your phone dies, or you're at home during a power outage, your car's battery is a reliable power source to tap into.
Sometimes these are wired for a specific kind of phone or connection. That's fine -- but if you can find one that uses a generic USB socket, you'll be able to power a wider range of devices.
The model shown here, the Trans4m Multi Power Source 4 Way
, has the added advantage of allowing you to recharge your phone from a pair of AS batteries -- which brings us to our next option...