What's a harmonograph, you ask? Well, it's something like a Spirograph for grown-ups. A harmonograph uses a construction with pendulums to draw pretty patterns.
One particular incarnation uses two pendulums of nearly equal length, each about two metres. To the right pendulum, a slowly revolving disk is attached, which is driven by a small electrical motor. The drawing paper is taped onto the disk. To the left pendulum, an arm is attached with the pen at the end. When the pressure and thus the friction of the pen is sufficiently low, the pendulums can keep swinging for tens of minutes.
For learning and fun, I replicated this harmonograph in JavaScript with the HTML5 canvas element. The pendulum motion is approximated by a sine function, and friction by an exponential, but apart from that it should be pretty physically accurate.
Without further ado, I present to you this Harmonograph in JavaScript. Usage should be self-evident. Although some input validation is done, you will be able to break it with strange values.
You can save and bookmark your creations in the form as links. You can also export images in PNG and SVG format. Enjoy!