I've been using Firebug for "debugging" web pages for a while now. I came across a sort of equivalent for IE - the IE Developer toolbar. Update: see below for IE 8 and above.
It's not quite as slick. I turned on the outlining of DIV elements and it spent the next half hour putting flickering boxes all over the screen until I killed IE. I won't do that again. At least IE7 remembers what was on each tab.
Update: This is by far and away my most popular blog post. So I figured I ought to update it for IE 8 and 9: IE 8 and 9 has built in developer tools, which look as good as firebug, though I haven't looked in depth. Press F12 to find them.
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