Take one Kinect and one projector, add suitable software and you have Beamatron, an immersive augmented reality environment that lets you interact with virtual objects and lets virtual objects interact with you.
Beamatron is an ambitious system from Microsoft Research that uses a projector and Kinect on a steerable platform on the ceiling.
The Kinect scans the room in 3D and the projector images virtual objects in the room that move in ways that take account of the room's layout. In the video below you will see a toy car being driven around the room reacting to obstacles as if the car and the obstacles were real. For example it travels up ramps and can be driven through a wall.
The 3D model is also used to correct the projected image so that it looks right to the observer in the room; i.e. it takes account of the shape of the surface on which it is projected.
The complete system was presented and described in a paper at the Symposium on User Interface Software and Technology (UIST) 2012 - the only problem is that ACM journals are behind a paywall so you can't read it unless you have library access or are prepared to pay an excessive amount.
What could you use it for?
The obvious candidate is for games but would you really want a Beamatron hanging from your ceiling? Education is a much more likely candidate but the Beamatron team suggest a much more radical application. Suppose your really important messages followed you around the room and kept on putting themselves in front of you? That urgent tweet or the tax demand you have been trying to avoid might just have a way to keep themselves in your view.