This LEGO walker robot was invented as a platform for a humanoid robotic SUMO contest, but it proved to be too lightweight for that purpose. The robot can walk and turn using just two motors, so the third could be used for moving the arms. The robot walks by swinging the weight (the NXT) from side to side at a rate that is the natural pulsation of the mechanical system, an inverted pendulum: the resonance allows using only a few energy to get wide oscillations. To keep the legs movement in sync with the oscillation of the NXT, the angle of swinging motor is measured. A touch sensor acts as limit switch to reset the upper body at the beginning of the program.
Download the building instructions and the NXC program from here.