The BumpTop concept is the same as it is in Windows, which is to say it's a 3D sandbox of a regular desktop. Icons can be placed on the floor - the main desktop surface - or any one of its four walls, which can be viewed from the top down, or head on. Icons can be stacked, literally, into piles which can then be previewed in a variety of ways, and interact with one another as solid objects would. If you throw one, it has momentum. If you throw one into another one, they collide.
Execution on Mac feels a bit smoother than on PC, mainly because the multitouch gestures, first seen in the Windows 7 version, work so well with MacBooks' glass trackpads. The metaphor is fun, if not immediately practical, and the performance penalty is minimal - my install idles at around 1% CPU use and 90MB of RAM after a few minutes of use.
You can try BumpTop for free, while a $US30 Pro version adds multitouch support, instant search and a few extra gestures. As an interesting tech demo it's definitely worth a download, and for what it's worth, it blends into my computer habits pretty well, to the point that I forget about it until I run into my desktop (which to be honest, doesn't happen a whole lot), only to be pleasantly, if slightly, surprised. But $US30? That feels little steep for what amounts to giving a single folder in your OS a makeover. [BumpTop]