[ks]

Microsoft has released a program called "Web SuperPreview". It enables you to compare pages both in IE6 and IE7/8.

http://www.microsoft.com/downloads/details.aspx?FamilyID=8e6...

[duskwuff]

The performance is kind of "meh", though, especially in VMWare (where it's practically unusable). I personally prefer IETester: http://www.my-debugbar.com/wiki/IETester/