We've covered othersolarchargers before, but this is certainly the smallest one to come across the desk. The Solio Bolt Solar Charger can be charged either via the sun (approximately a full day of sunlight charges the battery completely) or via a USB charger to a wall outlet (4.5 hours for a full charge).
Unlike other solar chargers, the Bolt is about the same size as a standard external battery. While it can certainly used in that manner, its strength lies in the ability to recharge in sunlight. If you're watching a baseball game, going camping for the weekend or simply working out in the yard, the Bolt can be charging while you enjoy the fresh air.
With an MSRP of $70, it's not significantly more expensive than a standard battery of comparable size and power. The main difference is, of course, that when the standard battery runs dry, you're done.