$13 tamiyausa.com

This inexpensive kit includes two DC motors driving separate axles, providing independent control of speed and direction to left and right. It makes an ideal power plant for a small tracked or two-wheeled robot. The instructions include plenty of diagrams, and the kit is easily assembled using just a screwdriver and the bundled Allen key. It can be configured in both high-speed (58:1) and high-torque (203:1) gear ratios, and plays well with Tamiya’s Track and Wheel (#70100) and Universal Plate (#70157) sets to make a quick and versatile custom robot base. Solder some leads to the motors, add a controller, some sensors, and a power source, and you’re well on your way to a sweet little roaming ’bot.