I first heard about the BumpTop prototype back in 2006. It looks like they’ve refined it quite a bit since then… to the point where they’ve released it as an actual product. There’s a (limited) free version, and a Pro version that runs $29. Unfortunately, there’s no Mac version at the moment, but there is a little checkbox to let the developers know you want one, on the Downloads page.
Damn was it 2006? The system of interaction is great, and has been widely implemented, but maybe its a little too bulky for everyday use. But I would like to pile files that way!