This one's great -- step-by-step instructions by Ned Batchelder on how to fold six business cards into an origami cube!

If that isn't enough for you, Batchelder points to the web site of Dr. Jeannie Mosley, who organizes the Business Card Menger Sponge Project -- an attempt to build a "depth 3 approximation to Menger's Sponge" out of 66,048 business cards. A Menger's Sponge is an enormously fractal cube -- i.e. a cube made out of tons of other cubes:

Menger's sponge (sometimes wrongly called Sierpinski's Sponge) is a fractal solid that can be described as follows. Take a cube, divide it into 27 = 3 x 3 x 3 smaller cubes of the same size and remove the cube in the center and the six cubes that share faces with it. You are left with the eight small corner cubes and twelve small edge cubes holding them together. Now, imagine repeating this process on each of the remaining 20 cubes. Repeat again. And again ...

To develop this, Mosley designed a bunch of angular solids you can make out of business cards, and assemble into the massive uber-cube. You can check them out here, and then see what they look like when assembled into a "depth 2" Menger's sponge -- she actually built one of these using 3,456 business cards, and took a picture of it.

An interesting question: Are business cards actually strong enough to support the weight of a massive "depth 3" cube? She thinks so, and has the math to prove it.

You simply cannot get weirder than this.

(Thanks to Boing Boing for finding this one!)