I spent a Sunday afternoon a couple of weeks ago putting together a proof of concept for a Kinect-based drum machine. Wave to get the camera's attention, and then make beats by moving your fist in the air! It works best if you pretend to hold a drum stick in your hand while playing.
This was written in VS 2010 using OpenNI + NITE and WPF (thanks to this thread on the OpenNI forum for providing the example I needed to get started). NOTE: in order to use the sample players, you must have Windows Media Player installed on your system.
Download the code here
Update: I've updated the demo to use skeleton tracking. In other words: now you can use BOTH hands!