Tonight I was looking for a tool which would allow me to do real time editing of CSS in IE. (Think Firebug for IE.) I've blogged about the IE Developer Toolbar in the past, but to my knowledge it doesn't allow you to modify HTML or CSS like Firebug. Anyway, a quick Google search lead me to IE Watch. At $169 the tool isn't cheap, but if you are in an IE only shop then the tool is probably well worth the price.
For a list of other web developer tools for IE check out From Microsoft TechEd 2007: Web Development Tools for Internet Explorer on the IE Blog.