Back when I was in college, I remember seeing something about an IR field with a projector mounted vertically on a ceiling (so it pointed down at a table). Some software developer had made a program that allowed him to touch anywhere on the table and have it interpreted as a mouse click. He had a foot pedal to activate for a right mouse-click and could even draw bounding boxes on-screen.

He was using, of all things, Warcraft III to demonstrate the program.