"BumpTop is a fresh and engaging new way to interact with your computer desktop. You can pile and toss documents like on a real desk. Break free from the rigid and mechanical style of standard point-and-click desktops. Interact by pushing, pulling and piling documents with elegant, self revealing gestures. BumpTop's stunning interface makes clever use of 3D presentation and smooth physics-based animations for an engaging, vivid user experience."
When it's available on Mac or PC, and/or available to both platforms by one license payment, I would seriously consider paying for it, given that it's below US$100.