IE developer toolbar
Although Firefox has had some nice
developer support for quite some time, Microsoft has released a nice new toolbar for IE, the
Internet Explorer Developer Toolbar Beta. This tool will show you your document object model, allow you to toggle display of table grids, and do other very cool things. I wonder how I lived without it for so long... (of course, you could say, "but Firefox could do it!", but how does that relate to three years ago?)