Responsive Design Testing Tool – Screenqueri.es
While creating a responsive layout, testing it against multiple popular resolutions is pretty important to decide/optimize the content blocks to be displayed in each.
Screenqueri.es is a free-to-use and web-based project for easily testing such layouts by defining the URL and selecting the mobile or tablet device.
After that, the application will load the URL and help you see "what's going on in each pixel" with the pixel-guided interface. And, you are not limited to pre-defined sizes but can change the width/height by playing with the ruler provided.
Also, it is possible to change the orientation with a single click.