This multi-layer display uses droplets of water as a projection medium. This way, several different projected areas can be seen for a not-quite-3D layering effect. The trick is in syncing up all aspects of the apparatus. There are three manifolds, each with 50 stainless steel needles for water drop production. A solenoid valve actuates the drops, a camera images them mid-air, and a computer syncs the images of the dots with a projector. In the video after the break you can see the SIGGRAPH 2010 presentation that includes a description of the process as well as action shots including a 3-layer version of Tetris.
[Thanks Mid_Sci via theEngineer]