One of the concepts that I find most interesting it the concept of 'Augmented Reality'. In short, this is computer rendered graphics overlaid on a live video feed of what you see. Look through your camera phone and see things that aren't there.
There are a few apps out for Android (I expect more as the concept grows and technology and software evolve).
SpekTrek - Ghost hunting game that identifies ghosts on the GPS, renders 3D ghosts in that area when viewed through the camera, then give you points for catching/shooting them.
Celest - Sounds like google sky, but overlays the data/labels on the image you see on the camera. Includes our local solar system objects (planets, moon, sun).
Wikitude World Browser - Overlays Wiki data via camera view and map view. See what points of interest are around.
While these apps are more of entertainment type applications, you can see how this concept could be applied to better things. Overlaying schematics on physical objects. Directions and Signs that dont exist in reality. Information that normally isnt available and/or referenced against the real world.
One step further and this concept could be applied to glasses that you would wear to give you a more permanent display of augmented reality. Think John Carpenter's 'They Live' without all the evil alien stuff.