Have you ever think of this question? Yes, I always ask this question because it's critical to test your web application in different browsers, different resolutions and different brands before sending it to client. However, I haven't heard any real solution after the release of IE 7.
This question only happened in IE 7? No, but we can install various browsers and power toy for switching different browsers easily. However, it's a pain if you want to test your web page in IE6 after your installation of IE7. I searched for long time and try to find the solution: Tweaking by RegEdit, install Virtual PC and different environments...etc. All of them are not good and easy.
Until today, my UI designer share a little application by TredoSoft with me.
This tool is a small but amazing app, which enable you to install different versions of IE (who care about IE3 and IE4 by the way?)