If you've ever wanted to interact with a computer by waving your hands in the air, you can with a Wii remote, some IR leds, and some reflective tape. The IR camera in the front of the Wii remote is sensitive to sources of infrared light.
If you create an IR LED array around the Wiimote and use some retro-reflective tape, any object you can put the tape on will reflect enough infrared light to be seen. This video will walk you through the steps of how that works and what you can do with it.
The demo grid program is downloadable from my website at http://johnnylee.net