browserling - Live interactive cross-browser testing from your browser.
BrowserStack - BrowserStack is a software testing platform for developers to comprehensively test websites and mobile applications for quality.
Sauce Labs - Test mobile or web apps instantly across 700+ browser/OS/device platform combinations - without infrastructure setup.
Browsershots - Browsershots makes screenshots of your web design in different browsers.
CrossBrowserTesting - Browser Testing made simple! Run automated, visual, and manual tests on 1500+ real browsers and mobile devices. Test more browsers, in less time.
IE NetRenderer - Free online HTML tool that shows instantly how your website looks like in Microsoft Internet Explorer 7, 6 and 5.5.