A very light-weight (and just released) way to do this is to use Expression Web SuperPreview. It allows you to compare IE6 and IE7 (or IE6 and IE8+IE7-compatibility-mode) side-by-side. It's currently just a preview, but I've used it with good results. They're going to release a commercial version that enables side-by-side comparison of more browsers, but they say the IE-specific one will remain free forever.