Why do I love this app? It’s easy to use, and the two viewports update immediately to reflect URL changes in one of the frames. Not having to keep multiple browsers open at varying breakpoints and remembering to refresh each of them when I make a change has already saved me an immense amount of time. (You can also export a full screenshot of mobile viewport by clicking on the camera icon.)
While it comes with defaults for iOS, Windows Phone, Android and Blackberry, you can easily manage custom breakpoints and user agents within the app.