You may download it from here:
http://www.microsoft.com/downloads/details.aspx?FamilyID=E59C3964-672D-4511-BB3E-2D5E1DB91038&displaylang=en

I think it would be very helpful if you are working with the Web pages (for example, if you are developing a web page).

It's a plug-in of IE. And it let you explore through the DOM objects of the Web page you are visiting. And give you the ability to modify them and see the result as soon as you finished your modifications.

Features:

• Explore and modify the document object model (DOM) of a Web page.
• Locate and select specific elements on a Web page through a variety of techniques.
• Selectively disable Internet Explorer settings.
• View HTML object class names, ID's, and details such as link paths, tab index values, and access keys.
• Outline tables, table cells, images, or selected tags.
• Validate HTML, CSS, WAI, and RSS Web feed links.
• Display image dimensions, file sizes, path information, and alternate (ALT) text.
• Immediately resize the browser window to a new resolution.
• Selectively clear the browser cache and saved cookies. Choose from all objects or those associated with a given domain.
• Choose direct links to W3C specification references, the Internet Explorer team weblog (blog), and other resources.
• Display a fully featured design ruler to help accurately align objects on your pages.

So what are you waiting for? Have a try now, I promise that it will give you a big surprise! :)