Raspy Juice expansion board allows not only to juice up your RasPi through batteries or solar cells, but also gives you the features of a micro-controller for analog inputs and serial connections.

Of course, as per usual with small-batch boards for the Pi, finding out how to buy it is a royal pain in the backside. Eventually found this link to buy it for $42 (Singapore dollars?!? which is about £22 plus about £6 for shipping).

More details here: RasPy Juice: Rasperry Pi serial & servo control