The IE blog announced last month that after listening to developers feedback (or seeing what Firefox delivered as standard) they have created the IE Developer Toolbar.

• 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 800x600 or a custom size.
• 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. - Very handy

Though mainly a copy of what FF offer - its well worth the download and handy to have. Plus the ruler feature is pretty handy, and the DOM explorer is a nice way of drilling through your tags and elements to find any problems.