The Microsoft Internet Explorer Weblog has the details:
The developer community has asked for a long time: Where is the free developer toolbar for IE? We recognized the popularity of free IE tools like Fiddler and we listened to your feedback. I am glad to announce the next addition to our developer tool support: The IE dev toolbar. This tool will help developers to explore their HTML documents and understand everything about it.
At Chris’ talk at the PDC on Tuesday, he announced that we would have a beta version available very soon. This is now ready and available for download. It is designed to work on IE6 as well as IE7.