The Tamiya company produces a whole line of interesting mechanical components for model makers. These can be really useful for understanding key concepts better -- gear ratios, worm gears, and drive trains. Better still, they can integrated directly into your mechanical projects. Here are a few of my favorites:
Tamiya Tracked Vehicle Chassis Kit
Use this kit as a stand-alone or combine it with other accessories making a more complex machine with more capabilities. The gear box can be built with two possible gear ratios-Standard: 203.7:1 or High Speed 58.2:1. The motor is included.
Tamiya Planetary Gear Box Set
The Planetary Gear Set from Tamiya features a highly versatile system for reduction of high RPM electric motors to high-torque low RPM applications. RC-260 motor operates on 3 volts. Combining the gearboxes in various ways enables 8 different gear ratios -- 4:1, 5:1, 16:1, 20:1, 25:1, 80:1, 100:1, and 400:1.
Tamiya 70168 Double Gearbox L/R Independent 4-Speed
Suitable for use with remote controlled robots. Left/right independent design means you can set different gear ratios for either side. This set can be assembled for 1 of 4 different gear ratios -- 12.7: 1, 38.2:1, 114.7:1, and 344.2:1
Tamiya 72008 4-Speed Worm Gearbox H.E.
The gearbox features four gear ratios (84:1, 216:1, 555.4:1, 1428.2:1). The gear ratio can be easily adjusted by altering the middle and final gear attachment positions.
Tamiya 4WD Chassis Kit
The 4WD Chassis incorporates a four-wheel shaft-drive system similar to those in full size 4WD vehicles. A hex shaft transfers power from the center gear box to the outer gear box giving 4WD power.
Here's a full line of tools, materials, and kits offered by Tamiya.
[ Thanks Mike! ]