This is a very basic experiment (10 min hack of the demo that comes with the source) of the FLARToolkit.
Instructions

1. Give Flash access to your webcam
2. Print out this PDF (dont scale it in your printer)
3. Hold the PDF up to the camera

A much more sophisticated example of what is possible visit here
Source code for the FLAr Toolkit is here
The basic demo that we used is found here