There's a hidden feature in Windows Phone 8 that isn't very obvious.  If you want to switch between running applications, there's a keypress that will bring up a screen of all your running apps so you can select which one you want to jump to.

The big bonus is that you will re-enter the App at the same state it was in when you left to do something else.  For example, if you were on a particular page of a web site and needed to stop and calculate a tip, if you go back to the Start screen and launch Internet Explorer, it will go to the Home page.  Instead, using the App screen you go back to exactly where you were.