Oh, wow, the developer tool in IE8 is actually pretty impressive.
Back when I had IE7, I did a quick look into who had the better source viewer, because that can be rather important. I think firefox had some line numbers and syntax highlights, but otherwise wasn’t anything special. I completey forget Safari and Opera, but I think they had either text or something similar to firefox. IE6 and IE7 both had a Notepad window come up with plain text inside, and chrome had a pop-up window full of element attributes that would highlight parts of your webpage as you hovered over the different elements in the code. Needless to say, Chrome took the cake, and ate it, too. The other browsers were mad at it, but I guess they should have gotten pie, instead.
The new IE8 Developer Tool lets you change between IE6, IE7, and IE8 rendering modes in two clicks, sends your code off to the W3C validation tool, resizes the window, creates rulers, view the document with a collapsible tree view and highlighted syntax, and more.
Most importantly: not only can you see every element and attribute in both HTML and CSS, but you can disable CSS or disable individual styles.