Expression Web SuperPreview for Windows Internet Explorer is the quite long name of a great tool for testing web page layouts in different browsers (I'm just gonna refer to it as SuperPreview form now on).

SuperPreview allows you to compare how a web page is rendered in different browser versions. You can add several browser windows which you can display side by side or on top of each other. You can even add images to compare your web page with a Photoshop design mockup. The full version currently has support for these browsers: Internet explorer 6-8, Firefox and Safari.

Other features are: rulers and guidelines, zoom, browser size, DOM explorer, CSS properties and element highlighting.

SuperPreview is a part of Microsoft Expression Web but there is a free stand-alone version of SuperPreview that only has support for the Internet Explorer browsers.

More info on SuperPreview
Download the stand-alone version
More info on Micorsoft Expression