Available now on the Mac App Store.
It downloads app icons from the iPhone/iPad top 400 charts for any region, category, or chart type, and drops them from the top of the screen laid out in a colorful rainbow.
It can also be set to display the more popular apps more often, can be set up to run as a screensaver, and supports multiple screens.
Appfall is also a great way to find new apps. Point at any icon to see the app's name, then click to see more information and a link to the App Store.