Researchers at Nokia's laboratory in Tampere, Finland, developed a touchscreen interface that can be used on blocks of ice:
A near-infrared light source mounted behind the "screen" bathes it in invisible light, and an array of near-infrared cameras, also behind the wall, are focused on the front surface. A hand placed on the ice reflects the light towards the camera array and the signal each camera receives helps a nearby PC establish the hand's position, size and motion. The PC is also connected to a projector, which uses the data to project imagery - such as flames - beneath the user's hand.
Link (registration required) via DVICE
Previously: Water-Based Touchscreen