SuperPreview is disappointing, IMHO. It shows screenshots of the various browser's renderings, but they aren't interactive. Since a lot of cross-browser bugs are JS related that doesn't really help.

Xenocode's browser sandbox on the other hand is great: http://www.xenocode.com/Browsers/

It lets you run all versions of IE, Opera, Firefox, and Safari without having to install them. They were also sharp and included the IE Developer Toolbar in the packages. Highly recommended.