Universal USB/DC/Solar Lithium Ion/Polymer charger - bq24074
The Adafruit Universal USB / DC / Solar Lithium Ion/Polymer Charger can use USB, DC or Solar power, with a wide 5-10V input voltage range. The charger chip is super smart, and will reduce the current draw if the input voltage starts to dip under 4.5V, making it a perfect near-MPPT solar charger that you can use with a wide range of panels.
The bq24074 which powers this design is great for solar charging, and will automatically draw the most current possible from the panel in any light condition.
More information is available on the website of Adafruit.