"IETester is a free WebBrowser that allows you to have the rendering and javascript engines of IE8 beta 1, IE7 IE 6 and IE5.5 on Vista and XP, as well as the installed IE in the same process. "
This site hosts the Uniform Event Model Javascript which makes IE handle events the same way as FireFox, Opera or any other browser which implements the W3C DOM3 Event recommendations. See the about seection for more information.