I used Processing to analyze the accelerometer data from the Emotiv headset and then sent that data via serial port to the arduino. Finally, the arduino controls two servos sitting perpendicular to each other on top of a cup. The straw shows what's going on.
About a year ago I prototyped a vertical wind turbine. It failed to work because the surface of each blade wasn't turning enough air. The turbine, even though it's plastic, ended up being too heavy. I used plywood for the base, which also contains the generator, and the turbine is made of styrene. Each blade was thermoformed to a curved surface.
While in undergrad, I spent a day making an inexpensive longboard. I used five layer plywood and applied varnish. The board is five feet in length and reinforced with steel. The bolts double up as grip on the top side. Since I already had bearings, the most expensive part of the longboard were the trucks. The pictures tell the rest of the story.