I would recommend something simple to start. How about this USB nerf rocket launcher from ThinkGeek: http://www.thinkgeek.com/geektoys/warfare/8a0f/ It's cheap ($25), cool, has horizontal and vertical movement plus firing, and the software is Windows-only. Not to mention that a Kinect + rocket launcher combo would be awesome.
I'd be happy to get you any of these devices if you wanted to document what you find with adafruit-like clarity. Even better would be an exchange to hook willing USB reverse engineers up with people willing to provide devices for those engineers. :)
Some people at our office purchased lots of USB gimmicks, like Rocket Launchers, on expenses because they had to test whether all those USB devices work with XenClient.
My secondary vote (and it's a selfish one) is this pedometer that only has Windows software: http://www.mattcutts.com/blog/my-favorite-pedometer-omron-hj... . Another possibility would the the Fitbit.
I'd be happy to get you any of these devices if you wanted to document what you find with adafruit-like clarity. Even better would be an exchange to hook willing USB reverse engineers up with people willing to provide devices for those engineers. :)