Laser Communicator

The coherent nature of laser light, combined with its low-divergence beam, makes it perfect for all sorts of fun projects. This is one you can do at home with simple, cheap parts. When you speak, the microphone picks up your voice and converts it into a signal that is used to vary the output of a  laser pointer.  The receiver is a solar cell attached to a piezoelectric speaker. With a pair of each, you can stay in communication as long as you have line of sight. It has the benefit of being hard to eavesdrop on as well. Pdf with instructions here. (Via crunchGear)

a similar project could be made to use the reflected light of a laser pointer off of a window to pick up vibrations and transmit sounds from distant buildings.