Formatted View Source
You don’t have to be a pro developer to love this one. Since the very beginning, IE has relied on the built-in Notepad editor to show page source. Notepad, it should be noted is not a source viewer.
In IE8, for the first time, the IE team itself took on ownership of the core development experience, making a deep investment in built-in developer tools. In the past, the developer experience belonged to the Visual Studio team (and the professional developer is still well-served by using that product for end-to-end development), but with the experience with the intern-developed Developer toolbar in IE7, the team realized that there is a place for a lightweight, built-in set of development tools.
A great side-effect of the deep work is that the team developed a great source viewer, and hooked it up as the default “View Source” app.
Bonus tip: If you have a better app (Notepad++ has been my standby for years) for source viewing/editing, don’t forget you can always change the editor to anything else by using the Programs tab in Internet Options. For my needs, however, I don’t think I’ll need anything more.