If you're in the field of site creation you will often need to check how a site looks and behaves in different browsers. Usually Firefox and internet explorer 7 and 6. But you can't install ie6 if you have ie7. A way around that is to use a great program, Multiple Ie. This program will install for you IE3 IE4.01 IE5 IE5.5 and IE6.

You can find a download link at the bottom of this page, and more information related to the project.