Thumbnails of running apps
Another way to switch among your running apps is to move your mouse pointer to the hot corner in the upper-left portion of the screen. When you see a thumbnail of your previous location, move the mouse pointer down. A black bar appears on the left showing thumbnails of your running apps. Click any to switch to it.
Note, however, that even if you've got multiple Desktop apps running, this bar displays only one Desktop-related thumbnail at a time -- the last app that was opened full screen or the Desktop itself.
You can also use the lower-left hot corner to toggle between the Start screen and the place you last were before going there.