WildCat is a four-legged robot being developed to run fast on all types of terrain. So far WildCat has run at about 16 mph on flat terrain using bounding and galloping gaits. The video shows WildCat's best performance so far. WildCat is being developed by Boston Dynamics with funding from DARPA's M3 program.
I have been following this project for a while, and am really excited to see where it's headed.