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.
With the link at the top, you can save and bookmark your creations or send them to your friends. Enjoy!
Update, 2017-01-22: Hosting is now done here on GitHub Pages. The old location redirects you. I also published the source code (“View Source” still works just fine, but it's now officially MIT licensed as well).