Aardvark firefox extension
I used to have a script in Homesite to turn table borders off and on so I could see how the layout of the web page I was making was being managed by all the nested tables. Now I have Aardvark, a firefox extension that does the equivalent for a page laid out with css. When I trigger Aardvark and hover the mouse over an element, I see a red rectangle around the element along with its id and class and element type. With hotkeys, I can remove the selected element, great for zapping ads, remove certain css attributes and see how this would affect the page you are viewing. Great for experimenting with css.
This extension is not only instructive, but fun and not half as fun as its going to be when they implement certain promised features.