jwz's link to this article about a woman who creates a huge origami structure out of business cards, inspired me to use my old business cards and make a smaller version of her 10-year project. The result is that I used 192 old businesscards and folded them in a structure that looks like a cube with its axes removed. This is called "Menger Sponge" and I made a Level 1 sponge in about 2 hours. It took the woman from MIT 9 years and more than 66000 cards to create a Level 3 sponge.

Pictures (with step-by-step instructions) are in my photo album as usual.