The funny physics sandbox of Garry's Mod and Kinect seem like a winning combination. If you don't think so, I'd recommend taking a look at the embedded video. I can only imagine what bizarre creations will come of this once it's in the hands of the public.
Garry Newman says the Kinect functionality will make it to GMod either this week or next in an update. For information on how this came about and a horrifying demonstration video thrown in for good measure, head over to his blog.