BrowserLab, a web-based service by Adobe, enables you to test websites on multiple browsers and OSs.

It is possible to create a custom browser set where the screenshots will be created only for those browsers (also helps speeding up the results).

Screenshots can be previewed in 2-up or onion-skin views which makes comparing multiple versions easier and x/y rulers can be used for sensitive measurements..

The service currently supports different Firefox, Safari and IE versions for Windows and Mac OS X. However, it is still in beta and new browsers are added within time.

BrowserLab is part of the Adobe CS Live and free for 1 year if you sign-up before April 30, 2011.

Website: http://www.adobe.com/go/browserlab_cslive