Adobe BrowerLab gives you the ability to preview and test your websites in multiple browsers and operating systems. Since building a website to be cross-browser compatible is one of the more annoying aspects of web design, this should be a valuable web app to all of us.
# CrossBrowserTesting.com allows website designers to test the cross browser compatibility of their website across different browsers and operating systems.
# Sometimes you need more than a picture. Test your AJAX and Javascript as well as the layout.
In my opinion, Xenocode Browsers is the ultimat tool for checking your website in different browsers. Xenocode Browsers allow you to lauch IE6, IE7, IE8, Firefox 2, Firefox 3, Google Chrome and Opera directly from the web. The only weak point: The service isn't available for Macs and GNU/Linux powered PCs.