Firebug: essential javascript debugging, and much more

Firebug 1.0, a tool for the live debugging editing, debugging, and monitoring of CSS, HTML, and JavaScript, was released on the 24th January 2007. This tool is essential for any web developer who needs to know exactly what is going on in a web page, especially with those hard to track down javascript errors. While it only works in firefox, there is a lite version for use in Internet Explorer, Opera, and Safari.

Get it here: http://www.getfirebug.com/