LV Boost
LVBoost is a boost converter that can take voltages as low as 0.5V and convert them into higher, more useful voltages like 3.3V or 5V. It has an exceptionally low quiescent draw - typically under .4mA. these features make LVBoost a great choice for powering projects from solar or other renewable energy sources.
From a 3V input, 500mA at 5V out is easily achievable, allowing you to charge nearly any portable device that meets the USB spec.
The output voltage is set with a small screw potentiometer on the side of the LVBoost.