I'm currently working on some tricky CSS coding and have the need to test multiple browser versions.  Of course, Firefox is no problem: very little has changed in CSS rendering from version to version.  Internet Explorer is a bit more tricky.  I've been running the v7 betas for months now and I really didn't want to uninstall v7 for a few quick CSS tests.  v7 fixes a lot of the rendering problems that previous version had, and I needed to verify that my page looked reasonable at least in IE 6 if not 5.5 and 5.01.  I was considering setting up multiple VPCs to test, but then I ran across an installer called Multiple IE.  This handy utility works as advertised - it allows you to have multiple versions of IE installed concurrently.  I have had some problems with versions 4.01 - 5.5 crashing on me, but that's not a problem for quick rendering tests.  All in all, I'm quite happy with the setup.  Alternatively, if you are still running IE 6 and would like to give IE 7 a spin without a full install, give IE Standalone a whirl.  A word of caution, Scott Hanselman - among others - has had some bad luck running running a similar setup.  I have yet to run into problems, but I'll be sure to post an update if the tide turns.