You can also use Firebug Lite, wich works on IE, Opera and Safari.
It's a Javascript implementation that you can load with a simple bookmarklet.
As SO doesn't allow Javascript, here is the bookmarklet source code (just copy paste to your browser location bar (always make sure it's safe before executing random javascript (In any case check the first link)))
javascript:var%20firebug=document.createElement('script');firebug.setAttribute('src','http://getfirebug.com/releases/lite/1.2/firebug-lite-compressed.js');document.body.appendChild(firebug);(function(){if(window.firebug.version){firebug.init();}else{setTimeout(arguments.callee);}})();void(firebug);
Firebug Lite supports all basic commands of Firebug.
