Testing responsive web design is tough job. Changing browser width and testing it across different browsers and devices is pain and adds to development time. These tools listed below will definitely aid you in testing your responsive website at multiple screen sizes.
Responsive Design Testing
Responsive Design Testing is a great tool by Matt Kersley to test responsive web designs. You can quickly test your designs in multiple screen widths. You can even share a link with someone by adding your URL to the end of the testing page.
Opera Mobile Emulator
Do your mobile development straight from your desktop, and pair it with Opera Dragonfly for advanced debugging.
Test any website at a multitude of common screen resolutions, including desktop monitors, tablets, and smaller mobile devices.
Great tool for responsive designs by Remy Sharp that helps you find the exact breakpoint widths to use in media queries.
A simple, useful and beautiful browser window resize app for Web designers and developers.