BrowserStack - BrowserStack is a software testing platform for developers to comprehensively test websites and mobile applications for quality.
Browshot - Browshot is a service to take screenshot of web pages. Use any of our mobile and desktop browsers. We provide a powerful API and libraries.
browserling - Live interactive cross-browser testing from your browser.
Browsershots - Browsershots makes screenshots of your web design in different browsers.
IE NetRenderer - Free online HTML tool that shows instantly how your website looks like in Microsoft Internet Explorer 7, 6 and 5.5.
Selenium - Selenium automates browsers. That's it! What you do with that power is entirely up to you. Primarily, it is for automating web applications for testing purposes, but is certainly not limited to just that.