[Update March 18, 2009 - see my new post about a new feature called SuperPreview for testing your website in multiple versions of IE: http://blogs.msdn.com/anna/archive/2009/03/18/test-your-sites-in-different-versions-of-internet-explorer-part-2-introducing-superpreview-beta.aspx . Now back to the original post ....]
Many web developers that I know rely on multiple physical machines or virtual environments to test their websites in different versions of Internet Explorer. Seems to be one of the most time consuming tasks that a web developer does…not just tweaking their markup to ensure cross-browser compatibility, but also the manual process of pulling up each page of their website in different browsers, browser versions, and on different platforms to boot.
Some alternatives exist for testing your website in different versions of Internet Explorer, including: