What a great idea. Some brits have built a light powered entirely by gravity. The goal is to replace kerosene lamps in Africa and India, and they initially looked at solar, but decided this was less expensive and more reliable. The target cost is $5 for third world end-users, and I suspect with some markup they could sell it elsewhere for emergency and niche use and fund their third world efforts.
In addition to providing a modest light source, it can power other small devices and (slowly) charge batteries.
Here's a link to the developers' appeal for funding, which succeeded. It includes video.