This app allows Hangouts to run in the background of your screen on any device you use. See my picture below and you will see a green hangout icon. When you click the icon, it activates your Hangouts dialog box.
Clicking on the Hangout icon allows the icon to overlay the desktop or web page you currently have open. You can drag the icon to re-position it on your screen. When you press the icon, your list of conversations will spring up.
Another awesome thing about this app. You can "pull" out people you chat with often and their pictures will appear above the Hangout icon. If you hover over the person, it shows the last chat or conversation you had with that person. AWESOME!
This a great app - why?? You don't have to go to your email or have it open to use the Hangout.
Here is a short video if you want to see the app in action.