FireBug extension for Firefox
FireBug a firefox extension that one day might challenge web developer as the most useful firefox extension in my toolbox. A heap of the tools to poke, prod, and monitor your JavaScript, CSS, HTML and Ajax including a debugger, error console, command line, and a variety of inspectors. I can not get it to do everything it is supposed to like live editting of the HTML, but even so the inspectors are well worth the download.