MOZiE is an extremely light-weight, free Windows application that allows web designers the ability to compare page rendering in Mozilla and Internet Explorer by embedding Gecko -- via the Mozilla ActiveX control -- in an HTML page that sits above an iframe displaying MSHTML-rendering, running as an HTA. Download MOZiE, unzip and run.
MOZiE also contains other useful features such as: various screen-size emulations and the ability to launch active pages in browsers, such as Opera, directly from the interface.
MOZiE is (unfortunately) available only to Windows users running IE5+. Users must either register the Mozilla ActiveX control (if Mozilla 1.5+ is already installed) by simply opening the command prompt, going to the folder containing "mozctlx.dll" and typing "regsvr32 mozctlx.dll".