BetaNews: The toolbar works on Internet Explorer 6 and above on Windows XP and Vista operating systems. The toolbar will allow developers to perform such functions as validating CSS and HTML code, and previewing how pages appear in various screen resolutions. No release date was given for a final version. The toolbar beta can be downloaded from FileForum.
http://fileforum.betanews.com/detail/Internet_Explorer_Developer_Toolbar/1127145234/1