Millions of people live in shantytowns across the world, many in corrugated-iron-roofed shacks with no windows. This leaves the residents with the choice of living in complete darkness or running expensive electric bulbs (if electricity is even available to them).
Liter of Light has a solution which is so mind-bogglingly simple that it is pure brilliance:
Step 1: Take an old soda bottle and fill it with water, add a couple of drops of bleach to prevent algae and screw the lid back on.
Step 2: Snuggly fit bottle into custom-cut hole in roof and the plastic bottle will refract the sun's rays into the room below, delivering about 55 watts of light per bottle for up to 5 years!
Based on an idea apparently developed at MIT, social entrepreneur Illac Diaz's Liter of Light project has already brought light into 10,000 homes in the Philippines. Obviously the lights only work during the day but this is still a huge step toward increasing the standard of living for, to quote Diaz, "the bottom 90 percent of this country."