Scanlime's Beth modded a remote control vibrator, replacing the interface with an Arduino-based sonar controller that she can activate with any part of her body, playing it like a theremin. The result is pretty cool — it "closes the feedback loop" between the vibrator's intensity and the user's physical response. The post includes a detailed technical breakdown of the reverse-engineering steps that she used to work out how to hijack the control mechanism, and the steps that went into building the remote, including a 3D printed chassis. The plans are open source hardware (CC-BY-SA), and posted to Github.
This toy serves as a kind of analog bridge between two remote spaces: the column of ultrasonically-oscillating air in front of the remote, and whatever body part happens to be in contact with the vibrator. Touch that invisible space above the remote, and the vibrator touches you.
In fact, it does start to feel like there's a palpable object in space above the remote's sensors. Move your body close to it, and it reacts. Press into it lightly, or tease the edges. Flick your hand through it, or make graceful waves back and forth. You can use your whole body to touch it, almost like a big fuzzy vibrating cone floating in air.
If the sensor can see your body's rhythms, it responds in kind, effortlessly synchronizing to its frequency. This is exactly the sort of closed-loop control I was after.