This bracelet has an Arduino-powered ring of white LED's with a 3-axis accelerometer tilt sensor.
How many interactions/games can you think of with this platform?
There are 3 in the video:
Hand in the air: flashes (because at a party you want to signal that you want someone to talk to?).
Horizontal hand: always illuminates the top LED's whatever rotation your arm has ("smart flashlight")
Spins of the wrist: a blob spins around in the same direction and slows to a stop.
For a commercially produced inertial-sensing band keep an eye out on getymyo