The $50 interactive whiteboard

Take a \$40 Wiimote, an infrared emitting pen and you have the
makings of an interactive whiteboard (No Nintendo Wii required). Johnny
Chung Lee has been experimenting with the Wiimote
. The Wiimote
connects to the computer over bluetooth, and inside it is an infrared
camera that can track up to 4 LED sources.

Check out the video of it in action:

Now to find a source for some infrared penlights or make my own and try
this out. With projectors costing less than \$800-\$900 this would be a
very inexpensive way to bring interactive whiteboard technologies into
the classroom.