Microsoft Expression Web 4 SuperPreview 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.
The program is extremely easy to use. Just select the two browsers that you'd like to compare (IE6 and Firefox, say), enter a URL, and SuperPreview will render the page in both engines and then display them, side by side. You can optionally overlay the pages if you find that makes it easier to spot any errors. And if there is an issue, just clicking on the problematic page element will display it's tag, size, position and location in the DOM, helping you to quickly fix the problem.
Despite the name, you don't need a copy of Microsoft Expression Web to use SuperPreview - it's a stand-alone tool that works well on its own.
And even if you don't want to buy a copy of Expression Web when the 60-day trial expires, SuperPreview remains useful. It won't allow you to render pages in Firefox (or use the online service), any more, but you'll still be able to compare URLs in all the supported versions of IE.