A simple motor made by an wire and a magnet. First one uses a spiral wire, the second one a nicely shaped copper wire. They both use an AAA battery. The wire rotates due to the magnetic forces applied.
This video shows how to modify an optical mouse to make it work in mid air, without a table or mouse pad.
It also shows several application scenarios, including wall display interaction, Windows Media Center, slide presentation, and interactive video games.
Photosynth takes a large collection of photos of a place or object, analyzes them for similarities, and displays them in a reconstructed 3-Dimensional space.