We've seen biofeedback in video games before, most recently at GDC, where Valve discussed its application in Left 4 Dead 2, Portal 2 and other titles. This, however, is a little more brute force. And it's something you can build yourself, if you're so inclined.
By flexing his muscles, this guy has become an NES controller. It's a USB device that involves attaching EMG sensors to the left and right arms. The B button is a clench of the right hand; the A is a clench of the left. Left and right on the D-pad are the left and right biceps.
If you want to try it for yourself, Brian Kaminski, who holds a degree in biomedical engineering from N.C. State, has posted the plans on Instructables.