There’s something intrinsically cool about the design of these AR (Augmented Reality) tags. They have this cool 8-bit hieroglyph vibe to them that I really like.
The idea is that you place them on real-world object, and then a camera connected to a computer recognizes them and the perspective at which it sees them, and uses this info to render a virtual object at the proper angle, view and perspective.
The following image from this extensive ARTag information page gives you a decent idea of how these work:
I discovered the enigmatic AR tags via this article on BoingBoing about how the NASA Mars Curiosity rover has AR tags on the hull of the vehicle.