Streamlining the essential task of testing your website’s responsiveness, we present several top-tier free online resources for this purpose. 
This open-source tool, available on GitHub for local setup, enables you to visualize how a webpage will appear across different screen sizes. ResponsiveTest supports a wide range of devices, including iPhones, Blackberries, Samsung smartphones, Dell laptops, and more.
Responsinator accepts a URL and generates a series of device mockups with the webpage rendered within them, allowing you to gauge the user experience across various devices.
This tool offers a convenient feature with keyboard shortcuts (e.g., pressing a key to view the tablet preview), facilitating rapid toggling between device previews.
A straightforward Web tool for assessing site responsiveness, Am I Responsive? also provides a visual preview that can be effectively used in presentations and design discussions.
This bookmarklet allows you to resize your browser window to specific dimensions, such as an iPhone 5 or Amazon Kindle Fire, among Viewport Resizer’s 47 screen size presets.
resizeMyBrowser provides 15 default browser window sizes corresponding to popular devices (MacBook, iPad, etc.), enabling custom dimensions and showing the current window size.
Similar to others on the list, Screenfly takes a URL and offers a preview of the webpage across various predefined screen dimensions.
Designmodo’s responsive design testing tool is a valuable asset during the design or debugging of responsive breakpoints.
Like Responsinator, this tool from pixeltuner.de allows you to input a URL, which is then displayed within multiple device mockups.
responsivepx is compatible with local files, making it practical for working on responsive design projects on your computer.While these responsive design testing tools share a common approach, the choice between them often boils down to personal preference. I suggest experimenting with each and selecting the one that best suits your needs. What is your preferred responsive design testing tool? Share your thoughts in the comments below.
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