A nice rolling linefollower.
Line Follower
                A Line Following
                Robot:
                
                After reading both
                of the
                new books about Lego Mindstorms,
                I realized I had never created
                a line following robot.
                So, I built a generalized chassis
                to play with the idea.
                
                I used two light
                sensors and two motors.
                I mounted the lights sensors one facing
                down for line following and another facing
                out for any IR radar applications.
                
                The motor setup is
                pretty simple, but
                very stable and fast. I geared the
                motors down substantially for line
                following.
                Here is some sample code from
                Dave Baum’s
                Definitive Guide to Lego Robots used
                with
                this robot.
                Here is the mpeg movie of the
                robot
                that shows the line following in action.