I just read this great article by Mark Voermans: Expression Web SuperPreview: Look how your site renders in various Browsers

I don’t know about you guys, but I’ve been waiting for this for a long, LONG time. I’ll give it a go this afternoon and let you know what I think…

Update
This first beta version only allows you to preview your site in different versions of Internet Explorer. We will have to wait for the full feature set until the final release of this tool…

Taken from the SuperPreview site:

This free download will allow you to compare renderings of IE6 with whatever other version of IE you have installed on your machine. If you have installed IE8, you’ll be able to compare IE6, IE8 and IE8 running in IE7 compatibility mode, side-by-side.