Sun has a 3-D desktop demo called Project Looking Glass that allows you to treat windows as if there were three-dimensional, as well as add other common effects such as transparency. The demo was mainly designed to show off the performance of the system by running two videos simultaneously while rotating the windows, which also had little dock thumbnails that played the video as well. The one use of the 3-D system that I thought was interesting was that you now get the notion of the backside of a window. In the demo, they showed the example of being able to flip the window around and leave a post-it, view properties of a video, or watch the video backwards if you so chose. This seemed to be an elegant way of including metadata about an object, as it fits well with real-world metaphors.
(via Mac Rumors)