BigDog is a quadraped robot being developed by Boston Dynamics for possible military usage. It is by far the most advanced quadraped around. The site has more mechanical details and a video! About 30 seconds into the video, he kicks BigDog and it stays balanced!

So far, BigDog has trotted at 3.3 mph, climbed a 35 degree slope and carried a 120 lb load. BigDog is being developed by Boston Dynamics with help from Foster Miller, the Jet Propulsion Laboratory, and the Harvard University Concord Field Station.Development is funded by the DARPA Defense Sciences Office.