ProtoFluid

ProtoFluid is a web-based prototyping tool that lets you test your website prototypes in various screen sizes and resolutions. Just type in the URL, select the device (or any custom dimensions) and press launch. Since it is a web-based tool, it also lets you use other extensions like FireBug.

Responsive.Is

Responsive.Is is created by TypeCast another browser emulator tool, which you can use to test your responsive design. Just type in a URL, and it will automatically change its size depending on the device you choose.

Responsivepx.Com

ResponsivePx is an awesome tool for testing your responsive website design. The main feature that distinguishes it from others, is its capability to resize the website pixel-by-pixel. This awesome feature, will let you identify the breakpoints and also test how the CSS media queries are working in your site.

Responsive Web Design Testing Tool

An awesome testing tool, that can allow you to view your responsive website in various screen sizes simultaneously in a single screen, while you build or design them. I like this tool mainly because it shows all the screen resolutions side-by-side which makes it easier for debugging.

ReView.Js

ReView is a dynamic viewport system, developed in pure JavaScript, which gives you a fantastic viewing experience for your responsive Web design.

Screenfly

Screenfly by QuirkTools, is an amazing tool that 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.

Screenqueri.es

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.

The Responsinator

Test your site in various devices from an iPhone and iPad, to a Kindle and on Android on the Responsinator. It also shows your site both in portrait and landscape mode. I like this tool much more because of the outlines of the devices displayed on the page, which brings more meaning to the whole process.
Source: http://www.hongkiat.com/blog/rwd-tools/