I'm sure you've heard of TrackIR, which can do head tracking with some head-mounted reflectors. But without mounting anything to your head, the tobii eye tracke

I'm sure you've heard of TrackIR, which can do head tracking with some head-mounted reflectors. But without mounting anything to your head, the tobii eye tracker can do both head and eye tracking completely passively. It seems like adding hand/finger gesture controls, and peripherals like stylus, mouse, etc... would be an obvious next step, but it doesn't look like they or anyone else is doing that.