If you don't know what Augmented Reality is then I'll tell you.
It's magic!
Well nearly, it's webcam magic. There's open source software available called ARToolkit in c++ or there's FLARToolkit for the Flash platform which is really taking off at the moment. In a month or two you'll probably be sick of it. This demo was put together by John Lindquist with some help from Seb Lee-Delisle and it rocks! Check out more AR examples on John's blog, Seb's blog or at squidder.
