Things to do with cardboard.
Pragmatic: A company named Cardboard Frame uses the stuff for a low-cost artwork shipping and framing system. Ship your prints without getting them banged up, and they're ready-to-hang right out of the box.
Fantastic: An unnamed architect stacked 360 layers of cardboard into a 2.5 meter cube, then carved a sound-insulated listening room out of it, complete with stereo and integrated wiring. And the layers aren't even glued; the weight of the sheets hold themselves in place!