The video above is pretty mesmerizing. In it 32 metronomes are set to the same tempo, and started randomly. Basically they are all ticking away at the same speed, but they are out of phase with each other. Over time, miraculously, they all align with each other.
OK, it isn't really a miracle at all. It turns out that the blue foam that they are all sitting on is set up to be able to move left and right. As the pendulums swing back and forth, they cause the whole platform to move slightly. It wiggles in opposition to the average of all the swinging arms (equally and oppositely to the sum of their motions). This motion, in turn, affects the pendulums. Those that are moving opposite to the platform's motion find themselves getting a little bost, while those moving with the platform slow down slightly. Over time, this causes them all to line up. Pretty darned cool if you ask me. Physics rocks.