My grandfather send me this video last week:
Harvard Natural Sciences Lecture Demonstrations: Pendulum Waves.
Fifteen pendulums are tuned to oscillate at 51 cycles-per-minute to 65
cycles-per-minutes. It creates interesting aliasing effects for one
minute, after which the pendulums come back into sync and the system
starts over again.
I thought it was interesting enough to simulate in software, so I made
an applet for it,
That should be exactly in sync with the video, so you can watch them
side-by-side and see the same patterns.