Adobe BrowserLab is a new online service that will help you test the layout of your web pages across a variety of web browsers and operating systems.
Comparing web design in Firefox 3.0 & IE 7.0
You can use the Browser Lab service to view the design of your site in Firefox, Internet Explorer and Safari on both Mac and Windows. BrowserLab works by taking screen shots of your web pages in different browsers on different platforms and then displays these screenshot images side by side for easy comparison.
Though the service doesn’t support Chrome, Opera and some Linux specific browsers, the advantage is that there’s no waiting time and it also offers skinning so you can place renderings one over the other to quickly spot the differences.
Skinning superimposes one screen shot over another
BrowserLab can integrate with Dreamweaver CS4 so you can test web page designs from within the WYSIWYG HTML editor without having to publish pages on to an external web server.
Related: How to Completely Test your Website