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.
Firebug Lite is basically a JavaScript library which can be embedded in a web page or through bookmarklet can be used in various ways, you can embed the script files into your web page or if you don't want to download script files then you can use them directly via net.
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.