This awesome pocket-sized solar USB charger with battery backup can be used to charge cell phones, iPods, or anything else that can be plugged in via USB.
It is built into an Altoids tin, so it is the perfect portable size and a great low-cost alternative to store-bought versions of similar chargers.
This particular charger uses a rechargeable 9-volt battery, so will require at least a 9-volt solar panel, which can be purchased online for about $10.
The schematic below was created by bluepapercanoe (video below) to simplify the process of building this.
Here is a full tutorial on how to make this clever little charger using inexpensive materials.