Boston Dynamics has created BigDog the robotic pack mule that they claim is "the most advanced quadruped robot on Earth!" BigDog probably is as they claim as it can carry four infantry backpacks while keeping its balance over various terrains. In a video on the BDI site (28MB WMV but worth seeing) you can see BigDog walking through mud, over slippery snow, over rocks, up a hill, and even keeps itself from toppling over when it's being kicked! BigDog stands about waist high, can carry 165 lbs (75 kg), can walk through most terrain and can always keep its balance. The on-board computer controls locomotion, monitors external & internal sensors and keeps itself balanced. A gasoline engine powers BigDog's hydraulic system for actuating the hydraulically controlled legs. As seen, or heard rather, on the video, the robot is a little noisy, but it's clearly still in development and already quite an impressive rig!