While the previous tools each focus on a specific browser, these utilities are broader in their scope.
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() to work in all browsers and not raise JavaScript errors: http://www.getfirebug.com/lite.html.
Like Firebug Lite, NotobiBug is a cross-browser tool that covers some of the same ground as the more robust and refined Firebug. Its strength lies in its DOM and object inspection, though it has a capable console as well. The console and inspector can be invoked by including a reference to the Nitobi JavaScript file and calling nitobi.Debug.log(): http://www.nitobibug.com/.
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://github.com/kswedberg/jquery-tmbundle/.
This bookmarklet runs jQuery on web pages that don't already have it loaded, which allows us to experiment with jQuery on those sites in a console such as the one provided by Firebug or Safari's Develop menu: http://www.learningjquery.com/2009/04/better-stronger-safer-jquerify-bookmarklet.
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.charlesproxy.com/.
Fiddler is another useful HTTP debugging proxy with features similar to those in Charles. According to its site, Fiddler "includes a powerful event-based scripting subsystem, and can be extended using any .NET language": http://www.fiddler2.com/fiddler2/.
Sloppy is a Java-based web proxy that, according to its web site, "deliberately slows the transfer of data between client and server" in order to simulate loading web pages with a dial-up connection at various bandwidths: http://www.dallaway.com/sloppy/.
JS Bin is a collaborative JavaScript Debugging tool for rapid prototyping and sharing of scripts. It has a three-tab interface for writing JavaScript and HTML and viewing the output, and it automatically loads one of a handful of JavaScript libraries via a select list: http://jsbin.com/.
Bespin is a web-based, extensible code editor with collaboration features. It allows users to create an entire web site on Bespin's server and then deploy it to another server. As it uses the HTML5 Canvas element for rendering the code, only the most recent versions of Firefox, Safari, or Chrome are supported: https://bespin.mozilla.com/.
This Java-based web development IDE is free and cross-platform. Along with both standard and advanced code-editing features, it incorporates a full copy of the jQuery API documentation and has its own Firebug-based JavaScript debugger: http://www.aptana.com/.