Aside from being completely adorable, Fijibot is a completely autonomous robot that automatically seeks out light sources to charge his solar-powered battery.
He's built from a 1.5 liter Fiji water bottle, packed with an Arduino Uno, 6 volt solar panel, and an Arduino Proto Shield. His other parts are all off-the-shelf gear you could grab at RadioShack, while the wheels are from a discarded RC car.
Fijibot isn't a particularly useful robot, but it's still fun to watch him navigate from light source to light source trying desperately to stay alive.
If you're interested in making your own, check out the full tutorial by mikesoniat.