Online Responsive Webdesign Tester

What is a responsive web design?

"Responsive web design (RWD) is an approach to web design aimed at crafting sites to provide an optimal viewing experience—easy reading and navigation with a minimum of resizing, panning, and scrolling—across a wide range of devices (from desktop computer monitors to mobile phones)."
Source: Wikipedia

How do one normally test a  Responsive Web Design?

Just open the website in a browser on mobile device such as mobile phone or tablets.

What if you would like to test right from your browser on your computer where you are developing without the need to find out from another device? Yes, you can. 

Online Responsive Webdesign Tester
One of my favorite is "http://www.studiopress.com/responsive/" . It shows different device sizes in one single page.

• Open the link "http://www.studiopress.com/responsive/" 
• Enter your link
• Select "Width Only" or "Device Sizes"
• The view is shown for various sizes on a single page
•  If you have selected "Device Sizes" you will see for - 240 x 320 (small phone), 320 x 480 (iPhone), 480 x 640 (small tablet), 768 x 1024 (iPad - Portrait), 1024 x 768 (iPad - Landscape)

Find other online responsive webdesign testers

Test Responsive Webdesign right from Google Chrome using extensions:
http://techubber.blogspot.com/2015/03/how-to-test-responsive-web-design-in.html