I felt upon this website while checking out a few web development extensions for Firefox: http://ipinfo.info/netrenderer/
Basically, the site allows you to check what a website would look like under IE5, IE6, IE7 and IE8 Beta by simply taking a screenshot of the website actually opened in one of these browsers. What’s more, it allows you to see a made-up screenshot of the differences between IE6 and IE7 rendering.
This might well become part of my useful tools for web developments…
The Firefox extension can be found here: https://addons.mozilla.org/fr/firefox/addon/6455