Screenfly will enable you to view your responsive website in a variety of devices: Desktop, Tablet, Mobile and TV. It also has options to enable or disable scrolling or even to rotate the display.
More info: http://quirktools.com/screenfly
Resize Safari Extension
Quickly Test Website In Multiple Browser Resolutions, on Mac OS X
When doing web design, it’s critical to test how your page looks under different screen resolutions. Since the mobile web is exploding fast, you have to test against the mobile devices too. Resize is a Safari Extension that to make the process dead easy.
More info: http://resizesafari.com
Responsive Web Design Testing Tool
An useful testing tool, that can allow you to view your responsive website in various screen sizes simultaneously in a single screen.
More info: http://mattkersley.com/responsive
For Web Designers and Developers
A simple, useful and beautiful browser window resize app for Web designers and … and such), Inner means innerWindow size (interior of the browser window).
More info: http://resizemybrowser.com
Test your site in various devices from an iPhone and iPad, to a Kindle and on Android on the Responsinator.
More info: http://www.responsinator.com
Screenqueri.es is a pixel-perfect responsive design testing tool. Just enter any website address that you want to check, and this tool performs will show the website in various screen sizes according to device. You can also manually resize the size pixel-by-pixel to identify the breakpoints.