I've always enjoyed the Exposé feature that OS X has built-in for quickly viewing and switching windows, and Compiz does a wonderful job of replicating it on Linux. I've yet to find a suitable replacement for this functionality on Windows, however.

I'm looking for a utility that can temporarily scale down and tile all of my open windows (or previews of said windows) such that they all fit on my screen(s) at once without overlapping, and allow me to click on one to return my desktop and bring the selected window to the foreground.

* Activation must be quick and responsive, something on the order of 250ms-500ms from when I hit the button until all the windows are laid out. Animation is preferred, but faster response time will be the deciding factor
* Must be able to handle large numbers of windows. It should not bog down with 20-50 windows open, and will be running on suitably fast systems (Core i7 @ 4.4GHz, 24+ GB of ram, dedicated nVidia GPU)
* Does not need to be free
* Live previews of the windows is a plus, but I'd also be happy with thumbnails taken when the hotkey is pressed