Well its actually a line following mini-tank, cause were talking about a small one not the real size ones :-). This must be simplest line follower device because it uses just an analog circuit to follow the line, you might even call it old school, no modern microcontroller are used. The IR Emitter LED and Phototransistor are used for sensing white or black color. The LED emits IR light and the phototransistor receives IR light. Together they work well at see what type of light if any is reflected from a surface and since the line has different color than the rest of the surface we can detect where the line is. The guys over at PyroElectro are responsible for this project and they show you exactly how to build it from beginning to end with enough details and pictures for any DIYer to make their own.
Line Following Tank: [Link]