Making a business card cube is easy peasy - they stick together well too, here's an animated gif of the 6 stages.
I'm not sure what I'm going to do with all the cubes, but it helps pass the time when I'm on the phone.
Inspired by the Business Card Menger Sponge Project (a Geek Site of the Day)
Below is a movie I made of all the cubes I built piling up.