As mentioned before you can view the current Html DOM contents in IE using javascript 'xmp' trick, but recently Microsoft released the Internet Explorer Developer Tool beta (similar to the Firefox developer add-in) which allows 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 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.
I still tend to prefer Sizer to resize any window to an exact, predefined size. Also HttpWatch is superb for watching browser traffic, http codes and timings (but it is expensive), a cheaper alternative is Fiddler.
Another favourite I've used is WFetch from the IIS Resource Kit to debug http requests, security authentification issues and general low-level checks before coding them into .NET using the System.Net namespace often to programmically grab files.
Whilst we are talking about IE, saw this nice little history writeup with screenshots of the early versions.