Firefox had these kinda tools long b4, but here the MS developer toolbar:

“The Internet Explorer Developer Toolbar provides several features for exploring and understanding Web pages. These features enable you to:
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 and measure objects on your pages.“

Download

Many more tools:
http://dotnetpret.blogspot.com/2006/04/tools-tools-and-more-tools.html

Ref: by David Hayden http://davidhayden.com/blog/dave/archive/2006/04/30/2931.aspx