Termites
This is supposed to be an example of emergent behaviour. Each termite simply walks randomly around. If it bumps into a piece of wood, it picks it up and continues to randomly walk around. When it bumps into another piece of wood, the termite will put the piece it is carrying down next to that piece of wood and go back to wandering around.
There appears to be a problem with this however, since it doesn't work nearly as well as some other examples on the net. The termites sort of gather up the wood, but not in as tight piles as they should. I'll be continuing to work on this one.