Linus is a small robot that uses 5 infrared sensors to read the color of the ground beneath him. He is programmed to look for a black line and try to follow it to its endpoint. I tested Linus on a piece of white posterboard and used black electrical tape to design a track for him to follow.
This project was the first project (chapter 4) in my book Arduino Robotics, and I consider to be one of the easier chapters in the book. The core materials used in this project are (5) infrared emitter/detector pairs, an Arduino, (2) hobby servo motors (modified for continuous rotation), and optionally a motor-controller. Obviously you will need a battery pack, a frame of some sort, and some wheels... but those pieces are so ambiguous that I don't list them as specific parts.