Beautifully simple design: The plastic bottle light
Cut a hole in a developing world roof big enough for a plastic soda bottle to fit snugly into. Fill a bottle with water and a little bleach and set it halfway into the hole cap side up. Voila, you now have a daytime light bulb. The sunshine is diffused by the water in the bottle and kicks off the light equivalent to a 50 watt bulb that costs nothing to run. See how it works.