The Lizard robot
The Lizard robot model walks on four legs using a complex array of beam blocks on either side to control movement. Two independent drive motors are used, one on either side.
The Lizard robot is capable of recognizing “Stay” (yellow ball), “Walk” (blue ball), “Food” (green ball), and “Danger” (red ball). It can also can react to walls and objects, causing it to turn and head in a different direction. In the book, Laurens Valk explains how to use the NXT buttons as an override, enabling the changing of balls without triggering the ultrasonic sensor.
For more about the book, read my Geekend post Geek Gifts 2011: The Unofficial LEGO Mindstorms NXT 2.0 Inventor's Guide.