YourITronics

This AVR controlled autonomous robot is programmed to follow the brightest light in the room. It uses two wheels and a ping pong ball as a third wheel. Really simple components here and it runs on three AA batteries to sweeten the deal.

Robot follows the light with a ping pong ball: [Via] - [Link]

The counter contains only three inexpensive ICs (well, add a regulator and three transistors), and operates from 6 - 15V DC at about 25mA. The most expensive single item is the LCD display, which is an industry standard 16 x 2 dot matrix module, which can often be found used or at bargain prices. This isn’t a kitset, but the parts are easily obtained, and the circuit can be built using any prototyping technique, or you could design your own PCB. The prototype was built on a small commercial strip board.

AVR Frequency Counter: [Link]