I do love gadgets, linkages, clockworks, all that 19th-century cogs-and-wheels technology. So much easier to see than our 21st-century nanotubes and zillion transistors on a chip.

Sándor Kabai has written many Demonstrations illustrating such mechanisms. A few of his latest are:

Gyroscopic Joint	Straight Line as a Roulette
Lemniscate Plotting Device	Helicopter Tilt Control

What makes these interesting is that playing with the various parameters lets you vary both the geometric setup and the controls that make the mechanism go through its motion.

One in the same vein is from Stephan Heiss:

Mechanical Involute Gears

It shows how gears work. Amazing! You can vary the geometry to the point where the gears lock up and change gear ratios to get a really good understanding of what makes gears tick.