Kyle McDonald shows at Instructables.com  how to make your own cheap 3d desktop tracker.

Here’s a small video (non embeddable in my wordpress..)

You need :

• One Arduino board
• (3) 270k resistors
• (3) 10k resistors
• Solder
• Wire
• Aluminum foil
• Cardboard

It’s using capacitive touch sensing.

How cool is that ?