Let’s face the fact, because of this mixed UI with Metro and traditional desktop, the traditional way of app switching is no long apply, therefore, there is no more Alt+Tab to switch through the running applications in this new Windows 8 system. Instead, Windows 8 introduces a new feature called Switch List, see screenshot above, that pops up when called from the left edge of your screen, opposite from the Charm Bar.
To active using a keyboard
Use Win+Tab keyboard shortcut that used to bring up that very impressive 3D app switchboard in Windows 7 to cycle through the running apps from top down. You can also use Win+Shift+Tab to cycle through from bottom up.
To active using a mouse
Move cursor to the top left corner and slide down along the left board without clicking anything.
To active on a touch screen
Swipe in from outside of the left board on your screen and swipe right out without lifting up your finger. Swipe just like a “>” sign across the left board and the switch list will automatically slide out. This works on both Metro UI and Desktop, even when you are in the middle of Metro App.