Johnny Chung Lee, has some cool projects based on the Wiimote controller.  He uses the infrared ability of the Wiimote to track IR leds.  Using some custom software he's written (also available), he creates a digital whiteboard application.  The Wiimote can track 4 infrared sources, so there are some cool multitouch possibilities. 

He even helps you build your own IR enabled pen.  Check it out.
Visit his site for more info and software