I am really impressed by the tool. If you like me strugle to create a correct XHTML code and use many different CSS (where you could lose yourself easily), then this tool will give you the X-Ray look into your code. It's so easy to find what is wrong with your code and what style and CSS affect this particular element. The only sad thing about this tool is that you cannot edit the text, though you have the ability to launch an external editor. And the last sweet thing is that the cost is only $19.99. Oh, I almost forgotten about the list of great bookmarks, which will help you with HTML and CSS design (tutorials and tips mostly).
But you have to be Mac user to work with it.