Some of the best inventions aren't high tech, but technology adapted to the needs and resources of poor people in developing nations. When on Earth has a list of 10 of them, including these self-adjusting glasses.
The system is called Child ViSion. A pair will fit a child between the ages of 12 and 18 and withstand a lot of punishment. More importantly, they don't require an optometrist. The user can adjust them by twisting the rods on the side, which move the lenses. Once the user can see well, s/he removes the adjusters until it's time to tweak them again.
Read about 9 other great inventions to help the world's poorest at When on Earth.