Yes... a tree-climbing robot. After being introduced to the Arduino, Ben Katz used his imagination and metalwork skills to build a device that grips onto a tree and propels itself up the trunk. You can see it in action here:
After watching that a few times it still seems almost-impossible - but it worked. Amazing. A fair amount of metalwork has taken place and some revisions with the electronics, however you'd have to agree that this is an cool project. Once again it is great to see younger people use their imagination and learning to create such an imaginative device.
Furthermore, to Ben's credit he has documented his work very well, and you can follow along with it here.