How you make an invisible mouse? Answered
Wired has an article on Pranav Mistry's invisible mouse. It costs $20 to build and looks fairly simple. It consists of an infrared camera and an infrared laser with a line cap. I assume this line cap makes the laser fan out into a thin wedge of light that skims just above your desk's surface. The camera sees your fingers as hot spots and software tracks the movement of these hot spots.
Here is the original article:
It should be easy to make. Who want's to try it out and upload their code for us? I would if I could program.