Windows 7 includes Win+Tab function that gives you a 3D preview of running application, and lets you cycle through them. If you find the Win+Tab hotkey combination to be somewhat tiring, try out FlipCorner. It is a portable application that lets you enter the 3D preview mode by moving your cursor to a corner of your computer screen. You can set the corner of your choice, change the activation delay and define the size of reaction area to enable the function.
After installation, the application will sit in system tray. First, right-click its icon and choose Configure to set up and activate the application. You can select one or more corners to activate the 3D preview mode, set Reaction Area to define the area on each corner that can activate the function, and Activation Delay (the time you need to keep the mouse cursor in a corner to enable the 3D preview mode). Once settings are configured, click OK.
Now, move your mouse cursor to the defined corner to activate the 3D preview mode.
FlipCorner works on Windows Vista and Windows 7, provided you have Windows Aero enabled.