The JediBot, created by students at Stanford University, is a robotic arm that tests your skill with a lightsaber-ized foam sword. By using a Kinect, it's able to home in on its enemy's weapon and attack or defend accordingly.
Robots that can attack each other have already been demonstrated, but a system that can react to real-world human movements is pretty awesome. All they need to do is tweak it to move faster and hit harder, and it'd be more than a serious threat. [IEEE Spectrum]