IETester is a free WebBrowser that allows you to have the rendering and javascript engines of IE8 beta 1, IE7 IE 6 and IE5.5 on Vista and XP, as well as the installed IE in the same process.


Link:
http://www.my-debugbar.com/wiki/IETester/…

Check out these posts too:

  1. Test your website in IE6, 7 and 8 with SuperPreview
  2. Wanna give Windows Vista a test drive?
  3. Windows Vista Beta: A lot like Mac OS X
  4. You Can Install the Vista Upgrade Without XP
  5. Does your browser pass the ACID2 test?
  6. Test the Chrome OS in a Virtual PC
  7. Bizarre Websites On Which You Can Kill Time With Style