A Japanese computer scientist has come up with an interesting, soft, 3d touch screen that one can happily poke all day with no chance of sore fingers. Namely, a tank of water. Wait, what?
It works with a tank of water, and a webcam. The screen is projected to the bottom, and the camera can detect distortions in the water, and infer from them how the water was touched. From there, simple multitouch-screen software can send the input back to the computer, translating it into the manipulation of information.
While you'll never develop sore fingers, or carpal tunnel syndrome, from this setup, it does have the disadvantage of only working in one orientation (namely, the screen straight down from your perspective), and I can't imagine it's easy to type on. Still, Hack a Day described it as the most relaxing interface ever, which it effectively is.