Interactive GE
This has been going around, but I've finally had a chance to play with it. GE has a good demo of some very clever Flash technology. You print out a piece of paper, and then go to the web page, click a link, and hold the paper up to your web cam, and a little virtual world is superimposed in real time onto the scene. Here's a screen capture of me fooling around with it (and no, I don't have a sun in my chest):
If you move the paper around, the virtual world rescales and the viewpoint changes. It's pretty fun and worth a few minutes to fool around with it...