In researching an article written long ago, I came across a service that lets you see how your website and email designs appear in multiple platforms: Litmus. It’s a useful comparison to Browsercam. However, Browsercam does not test email designs (but does let you see your site design on mobile devices).
UPDATE (11/13/2009): Today there is no need to download software to view your application in different web browsers. Adobe’s BrowserLab currently is free and available anywhere with a modern browser. It’s easy to use and displays the full range of bad browsers, specifically, IE6 and IE7 on both Windows and Mac. But it also shows Chrome, Firefox, and Safari.
URLs:
http://litmusapp.com/
http://www.browsercam.com
https://browserlab.adobe.com