This has been posted on a number of sites already but it's well worth a one more to spread the word.
Joe Maddalone from Insert Title Web Designs has found a way to run multiple versions of IE on a single OS. To make the setup as simple as possible, Ryan Parman has provided the files for an easy 2 minute setup to eliminate any confusion.
Just unzip the files into a new directory and create a shortcut to iexplore.exe. The help->about window will still list your current install (IE 6.x) but everything will be rendered as the older browsers.