The Firebug extension for Firefox is indispensable for jQuery development:

Some of the features of Firebug are :

http://www.getfirebug.com/

This not only overlaps Firebug in the area of DOM inspection, but also contains tools for common tasks like cookie manipulation, form inspection, and page resizing. You can also use this toolbar to quickly and easily disable JavaScript for a site to ensure that functionality degrades gracefully when the user’s browser is less capable:

http://chrispederick.com/work/web-developer/

Venkman is the official JavaScript debugger for the Mozilla project. It provides a troubleshooting environment that is reminiscent of the GDB system for debugging programs that are written in other languages.

http://www.mozilla.org/projects/venkman/

Regular expressions for matching strings in JavaScript can be tricky to craft. This extension for Firefox allows easy experimentation with regular expressions using an interface for entering search text:

http://sebastianzartner.ath.cx/new/downloads/RExT/

The Developer Toolbar primarily provides a view of the DOM tree for a web page. Elements can be located visually, and modified on the fly with new CSS rules. It also provides other miscellaneous development aids, such as a ruler for measuring page elements:

http://www.microsoft.com/downloads/details.aspx?FamilyID=e59c3964-672d-4511-bb3e-2d5e1db91038

Microsoft’s Visual Studio package can be used to inspect and debug JavaScript code:

http://msdn.microsoft.com/vstudio/express/vwd/

To run the debugger interactively in the free version (Visual Web Developer Express), follow the process outlined here:

http://www.berniecode.com/blog/2007/03/08/how-to-debug-javascript-with-visual-web-developer-express/

The DebugBar provides a DOM inspector as well as a JavaScript console for debugging:

http://www.debugbar.com/

Memory leaks in JavaScript code can cause performance and stability issues for Internet Explorer. Drip helps to detect and isolate these memory issues:

http://Sourceforge.net/projects/ieleak/

Nightly builds of Safari include the ability to inspect individual page elements and collect information especially about the CSS rules that apply to each one.

http://trac.webkit.org/projects/webkit/wiki/Web%20Inspector

Drosera is the JavaScript debugger for Safari and other WebKit-driven applications. It enables breakpoints, variable watching, and an interactive console.

Though the Firebug extension itself is limited to the Firefox web browser, some of the features can be replicated by including the Firebug Lite script on the web page. This package simulates the Firebug console, including allowing calls to console.log() which usually causes JavaScript errors to be thrown in other browsers:

http://www.getfirebug.com/lite.html

TextMate jQuery Bundle

This extension for the popular Mac OS X text editor TextMate provides syntax highlighting for jQuery methods and selectors, code completion for methods, and a quick API reference from within your code. The bundle is also compatible with the E text editor for Windows:

http://www.learningjquery.com/2006/09/textmate-bundle-for-jquery

When developing AJAX-intensive applications, it can be useful to see exactly what data is being sent between the browser and the server. The Charles web debugging proxy displays all HTTP traffic between two points, including normal web requests, HTTPS traffic, Flash remoting, and AJAX responses:

http://www.xk72.com/charles/