Chameleon lets you quickly view the responsiveness of your web app in multiple device viewports.
Simultaneously review your site's behavior on a myriad of different screen sizes.
For testing, Chameleon opens your site in a new tab and shows you both the portrait and landscape orientation of each device.
Add and configure as many viewports and devices as you need.
View the source code on Github: http://stefanritter.com/chameleon/