Microsoft SuperPreview is a tool which was launched a few months ago at the MIX09 Developer Conference. This standalone visual debugging software makes it easier to migrate your sites from IE6 to IE7 to IE8 as well as compare how the site looks in different web browsers. The features found in this application were determined by asking web developers what sorts of things they needed in a browser comparison tool.
With this app, you can view pages side by side or even as an onion-skin overlay – a great feature that lets you stack a partially transparent browser on top of another to see the rendering differences. It also includes rulers, guides, and zoom/pan tools that help you navigate and identify differences in layout.
SuperPreview is available as a free 250 MB download from here. Note: requires .NET