Testing how your website looks and works in different browsers and different resolutions is very important for web designers. If your website only works correctly on huge widescreen monitors which many designers have then your visitors most likely will not see the same thing.
As a designer you are trying to give all your visitors the same experience and it shouldn't matter what browser you are using and what resolution you are viewing the site on.
Resize My Browser
This will change your browser to a number of different resolutions so you can see how your website looks at different sizes. It allows you to see how your website will look on different devices as it will point out the resolutions of certain devices.
Resize My Browser
Testsize is a website which is similar to above website Resize My Browser. It allows you to test any website in any browser size.
Preview mobile websites, design mockups, and local HTML on Mac or PC.
WebKit-based Adobe AIR app lets you preview mobile content on a variety of phone shells.
Visit any URL or drag and drop local HTML, Flash, or image files.
View the same site on multiple devices.
Export PNGs of device views - perfect for portfolios.
Quick, painless installation and upgrades via Adobe AIR.
Automated Browser Compatibility Testing
Browsera tests and reports cross-browser layout differences and scripting errors on your site.
Browsershots makes screenshots of your web design in different operating systems and browsers. It is a free, open-source, online web application that gives developers a convenient way to test their website's browser compatibility in one place. When you submit your web address, it will be added to the job queue. A number of distributed computers will open your web site in their browser. Then they will make screenshots and upload them to our central dedicated server for your review.
Find Me By IP - Browser Support
Discover how different browsers support the new features from HTML5 and CSS3.