This video from DARPA demonstrates their cheetah-inspired robot running on a treadmill, reaching a top speed and new world record of 18 mph. The robot, as of now, can't run without the support of a "boom-like device" guiding it, but the video is freaky enough that it attracted over half a million YouTube views in its first day on the web. 

On the YouTube page, the researchers describe the mechanism that powers the machine:

The robot's movements are patterned after those of fast-running animals in nature. The robot increases its stride and running speed by flexing and un-flexing its back on each step, much as an actual cheetah does.

The current version of the Cheetah robot runs on a laboratory treadmill where it is powered by an off-board hydraulic pump, and uses a boom-like device to keep it running in the center of the treadmill. Testing of a free-running prototype is planned for later this year.

For the sake of comparison, here is a video from Velocity Media Systems that shows a real live cheetah running in ultra slow motion:

For more videos from DARPA, visit their YouTube channel