This has been one of the most demonstrated features of Windows Vista and also one that I have been asked the most about how to use it.

Keep in mind that your computer must support the DWM for this feature to run.  If you are running Aero Glass right now, it will work perfectly for you.

Launch Flip 3D:

Just press and hold the Windows Key  + Tab.  Then use the scroll wheel on your mouse or the up and down keyboard arrows to navigate through the windows.  When the window you want is on top, release the Windows and Tab keys.