ARToolkit has been around for ages (10 years), but until quite recently was sadly locked up in C and C++ libraries. Recently it has been completely liberated and implemented in Flash, Processing, Java, C# and now runs on any toaster. The power of that is just huge. The usual first instinct with ARtoolkit is "Oh look at this virtual stuff on top of my real stuff. Then you have to suppress a yawn. But now you have all of these platforms and people with a completely new perspective start using ARToolkit in completely different ways.

The T-shirts are nice, but actually ARToolkit is not a good match, since it's not really a 2d code, but pretrained recognition (read the whole thing, Claus - they added 2d barcodes on top of the FLAR-pattern).

The calibrate and forget on a fixed surface works well.

The one I really think is brilliant is using non-recognition as the signal, with recognition as the steady state.

The ARToolkit code really benefits from coming out into the fresh air of Processing and Flash. New people with fresh energy to experiment and different ideas about what its for.