Eye tracking
Tobii's system really is pretty cool.
I splashed out earlier this year and bought an Alienware laptop with Tobii, and it transforms several things. Outside games, being able to look at something, touch the mouse pad, and have the cursor in the right place starts to feel natural after a short time. In fact, going back to a normal PC without it feels decidedly unnatural.
I haven't spent much time gaming with it yet (I'm working through Assassins Creed Black Flag at the moment, the newer AC games support it but that one doesn't), but the little bits I have felt brilliant.
I wouldn't suggest that it replaces all computer input devices, but it certainly works better than I ever imagined!