Microsoft Expression Web 4 is a visual debugging tool for web developers which can display the URL of your choice as though it was rendered by IE6, IE7, IE8, IE8's compatibility view and Firefox 3.6.

And if you subscribe to Microsoft's SuperPreview online service beta (just a matter of submitting and confirming your email address) then you'll also gain the ability to render pages in IE9, Safari 4 and 5 on the Mac, and Chrome, very useful for confirming that your sites display correctly for just about everyone.