spoon.net/browsers allows you to run multiple versions of IE (Internet Explorer) at the same time. You must visit the site using a version of IE to have the multiple versions launch.
You can now run IE8, IE7 and IE6 at the same time. No overly complicated installs required. Very simple to install and use. You do not have to install multiple versions of IE to get this service to run.
FYI – The reason someone would want to do this is for testing purposes.