If you have always missed the Alt-Tab in Android environment, you just found it. Just pull down the notification bar and switch to the previous active application. It is suitable for copy-paste operations or just to continue your previous job, or whatever you find helpful. After navigating away from your application its name and icon appear as a notification and you can easily get back to it just by selecting the notification. For your privacy, in setting panel you can choose an amount of time for hiding the previous active application.