With the release of Internet Explorer 7 (IE), web developers once again need to test multiple versions of IE. Unlike other browsers like FireFox, IE is a system component for the Windows operating system. This makes running multiple versions side-by-side very difficult.
Joe Maddalone first devised a method for running multiple versions of IE back in 2002. By leveraging DLL redirection, a technology that first became available in Widnows 2000, he was able to run both IE 5.5 and IE 6.
Manfred Staudinger further refined the technique by fixing the cookie support, adding version numbers to the browser windows, and getting conditional comments working.
Finally there is Yousif Al Saif's Multiple IE installer. Based on the earlier research, this setup package can install versions 3, 4.01, 5.01, 5.5, and 6.0 on a Windows machine. As this is a hack, users should be careful about installing this on their production machine and read the release notes and discussion very thoroughly.