When I was a kid in the late 70s, they were quite popular. The generator had a friction wheel that rested on the rear tire of the bicycle and turned when the tire turned. It did make pedeling somewhat more difficult.
Now I don't see them around anymore and I always thought they had a lot of potential for improvement. One big change would be to regulate the output so you wouldn't blow bulbs if moving too fast. Also perhaps a low power battery backup bulb. Or have the generator charge a battery, just like a car system. Of course they have high end rechargeable lights now, albeit quite expensive in some cases but I still think there would be a use for an improved bike generator light. True not for stop and go like mountain biking but for commuting road bikes, it would be an endless supply of renewable energy. I put mine to good use, commuting home from a late night restaurant job.
Anybody know if they're still around?