This is the lightest cheapest bicycle light I'm aware of. Furthermore, it does not take away pedalling energy and it´s quite aerodynamic.
I know that it is not very impressive, but I think it is really useful.
Step 1: Materials:
A pocket solar torch, avaliable in ebay for 2 €
A piece of inner tube or rubberband
Fix the torch in the bike, for instance with a M3 screw.
Make a knot on the piece of inner tube. This is needed because the torch only lights when the switch is pressed.
When you put the inner tube piece with the knot on the switch, the lights are on.
When you shift the position of the inner tube piece the lights are off and the battery is charged by the solar cell.