Web Development with Internet Explorer
Firebug rocks. Nothing, I mean nothing beats it for debug web pages. But what do you do for problems specific to IE (most notably IE 6) Here are a list of tools I use to help cause telling the user to get a real browser is not a solution
Internet Explorer Developer Toolbar
This toolbar provides some of the features of firebug.
Is a bookmarklet that let's you see information about elements on a page.
This may not be free in the future. Debug Bar from the same guy is also a very useful tool, but it's not free for professional use but probably worth the 59 euro if you do a lot of work with IE.
And finally something while not useful for debugging but damn useful in its own right is the Inline Search for IE.
Hope this helps somebody out.