Firebug Lite : Firebug for IE, Safari, and Opera
As a web developer I really love firebug, easy to use and really powerful web development tool. I always missed firebug when I test my pages with IE; although we have Developer Toolbar for IE I always find it tough to use when compared to Firebug. Most obvious reason is because firebug gets embedded in browser window instead of Developer toolbar which is a popup window and its really tough to move between your page and developer toolbar.
Firebug Lite comes to rescue :), now I can debug my pages in IE too as I used to do in Firefox.
I have used Firebug Lite bookmarklet and it works fine in IE 8 Beta(Emulate IE7 mode). Its not as powerful as Firebug and little slow but I am hopeful that its future versions will be more stable, feature rich and fast.