Robots, wonderful robots. My endeavores thus far in this field have been limited to a few BEAM bot kits, a bristle bot and some Lego Mindstorms. The BEAM bots are really neat. They have no programing at all, but can find lightsources and avoid shadows using nothing but capacitors and diodes for logic gates and sensors. Their capacitors are charged over time by the solar cells, and then discharge to one of two motors depending on the which photodiode is detecting more light. The twitching motors will walk them towards light.