Mosaic isn't intended to be a direct clone of the Windows 8 UI, and things could always change on either end, but the inspiration is clear. When you start up Mosaic, you have just a few tiles, but you can enter the Mosaic "store" to install new tiles, and then click the arrow in the upper right-hand corner to add them to your screen. You can get tiles that display the weather, your currently playing music, upcoming Google Calendar events, new Gmail messages and more. It obviously isn't ideal for doing all your computing, but it would actually make a very nice screensaver replacement, or "Start" page for simple email and calendar checking.
Mosaic is a free download for Windows only. If you don't like the fact that it's a separate program from the desktop, you can always bring the Metro UI to Windows 7 with these Rainmeter widgets instead.
Mosaic Desktop [Codeplex]