Firebug! The Firefox Plugin for web development and testing

[textile] Firebug... Too many features to list... but since it's a free firefox plugin, why not "check it out for yourself":http://www.getfirebug.com/?

Firebug is always just a keystroke away, but it never gets in your way. You can open Firebug in a separate window, or as a bar at the bottom of your browser. Firebug also gives you fine-grained control over which websites you want to enable it for.

!http://www.getfirebug.com/screenHome-js.gif!:http://www.getfirebug.com/

Debug and profile JavaScript Firebug includes a powerful JavaScript debugger that lets you pause execution at any time and have look at the state of the world. If your code is a little sluggish, use the JavaScript profiler to measure performance and find bottlenecks fast.

!http://www.getfirebug.com/screenHome-net.gif!:http://www.getfirebug.com/ And there's more... there are too many useful features to list for anyone developing or testing DHTML/AJAX web apps... !http://www.getfirebug.com/screenHome-using.gif!:http://www.getfirebug.com/ Oh... and did I mention it is opensource and therefore free (to use)! Check out "GetFirebug.com":http://www.getfirebug.com/