I spend much of my time wrestling with HTML either for web pages or the various weblications I work on, and I recently discovered the Internet Explorer Developer Toolbar.
It allows you to do things like browse the document DOM, label page images with their source URL (useful for things like Virtual Earth), and much more.
It's still in beta (when will we admit that all software is beta and move on?), and it does fall over occasionally. Also, it is not a completely passive probe - it can affect the document flow or page dyamics in things like Virtual Earth. However, it is still a very useful tool.