I have a power monkey which I keep with me in my commute bag at all times. Its very adaptable system, you can:
charge the power monkey from USB, or from a three pin with USB output, or the solar panel.
Charge the device from the power monkey or the three pin. Since multiple types of devices need only have the powertip changed you can support a lot more stuff with simple selection of power bits.
I also have a power chimp as another back up because that works on AA (or AAA with sleeves that come included) batteries. It gets a bit warm because of the simple electronics to get to 5v but its reliable and easy to use and to get AA batteries is generally quite easy.
7 dayshop are doing the chimp for £20 although I got mine for a bout £15 in one of their recent sales.
http://www.7dayshop.com/catalog/product_info.php?cPath=777_12&products_id=109355