Firebug in browsers other than Firefox!
I love developing web apps in Firefox because of the Developers Tool bar and Firebug add-ins. Unfortunately my current web development target is IE 6 (curse you client!), so I’ve been toiling in the underground sugar-mines with IE Developer Toolbar, which just isn’t as fun.
Firebug have released a “Firebug Lite”, a javascript library (which i’m testing and updating this post this week) which will give you a lot of Firebug functionality in other browsers!! Hooray!
The home page for this can be found at http://getfirebug.com/lite.html
UPDATE: Firebug Lite is pretty buggy itself in IE6.