By now, most people have heard of Ken Gracey's amazing 3.5" tall Penguin robots. He wrote an article on them in last month’s Servo magazine. The CNC drawings were published also and some in the robotics community have already built their own (for example, click here).
Max Kurz at the University of Houston has taken it one step further. He is studying actual penguins. Penguins use a passive-dynamic walking gait that may be useful for helping the disabled and for building walking machines. Max Kurz offers more details regarding the passive dynamics on his Biomechanics class page.
Now if Max Kurz could just shrink his ideas down into a little 4" robot.