I've always been a fan of anything that uses the concept of bone conduction. A friend who worked as a field medic for public protests years ago told me that he'd often diagnose and locate bone fractures by taking a tuning fork, striking it, and holding it against the limb in question–the sound would travel up and down the bone and cause a stronger 'sensation' (ouch!) wherever there was any sort of a break in continuity. Now, thanks to research being done at Carnegie Mellon and Microsoft, you can use this same basic technology to play tetris!
The video has a more in-depth demonstration, but the idea is based on the fact that our bodies are pretty effective conductors of minute acoustical information, so vibrations from something like a tap on the forearm or fingertips can be picked up by a bio-acoustic sensor positioned somewhere else along the arm. Because every part of the body is composed of specific combinations of different kinds of tissues with various densities, every location hypothetically has a signature resonance that can be tracked.
While it's still in development, they're already teaming the technology up with wearable pico projectors. I think it's really interesting for the future of AR, in terms of creating the ultimate ephemeral user-interface–Tablets are SO 2010.