Inspired by this YouTube video, this program simulates a laser beam shot into a collection of circular mirrors. The angle of the beam relative to the x-axis of the canvas changes very slowly, yet the behavior of the beam is very erratic. This property--small changes to the input of a system leading to huge changes in the output of that system--is the essence of chaos, at least from a mathematical perspective.
The initial angle of the laser is chosen randomly, so click "Refresh" to select a new one if the laser is stuck in a region of uninteresting behavior.
Here is a larger version that is more suited to desktop browsers.