Binary marble adding machine. I actually think it is really useful to know binary, and showing my kids this video was just one more way to impress upon them binary's usefulness. I'm going to try to build something like this out of lego, I think, though I don't imagine I'd be able to get it so beautiful.
Twist and Spin - Mesmerizing Handcrafted Oak and Brass Marble Machine Run. Steel Bearings on Wood and Brass Track