Javascript

General

Mozilla Developer Center: JavaScript (Mozilla): Documentation and articles about JavaScript from Mozilla.

JScript Center (MSDN): Microsoft's documentation for JScript, Microsoft's JavaScript implementation.

EMCA 262 (PDF): The core standard on which all JavaScript implementations are based

Migrate Apps from Internet Explorer to Mozilla (IBM): Article on differences between IE and Mozilla, particularly with JavaScript, which can help with cross-browser development.

comp.lang.javascript FAQ: Frequently Asked Questions file from the comp.lang.javascript Usenet group. Contains many common solutions to problems and JavaScript resources.

JavaScript Tutorials (w3schools.com): A self-paced JavaScript tutorial, examples, reference documentation, quizzes.

JavaScript Forums (webdeveloper.com): Get help with JavaScript here.

AJAX

Ajax: A New Approach to Web Applications (Adaptive Path): The original AJAX article which provides an overview of the technologies that make up AJAX: Asynchronous JavaScript + XML.

XMLHTTP (MSDN): API Documentation for Microsoft's XMLHTTP object.

XMLHttpRequest (MozillaZine Knowledge Base): API documentation for Mozilla's XMLHttpRequest

AJAX: Getting Started (Mozilla): AJAX basics, with examples.

Mastering AJAX: Part 1: Introduction: A detailed getting started article from IBM DeveloperWorks.

How do I parse a string with XML markup into an XML DOM document? : An FAQ from faqts.com with sample code for Internet Explorer and Mozilla.

XML for <SCRIPT>: An open source XML Parser for JavaScript, with lots of other XML development tools.

JSON

Introducing JSON (json.org): An overview and technical description of of JavaScript Object Notation, from its author.

JSON in JavaScript (json.org): Using JSON in JavaScript programming.

JSON: A Fat-Free Alternative to XML (json.org): A comparison of JSON and XML as a data representation format.

JSON Yahoo! Group: Discuss JSON here.

RSS To JSON Converter: a general-purpose tool to convert any RSS or Atom feed into JSON, by John Resig.

Debugging & Other Developer Tools

Scripting Debugging in Internet Explorer: Microsoft debuggers for Internet Explorer.

Venkman: A JavaScript debugger extension for Mozilla and Firefox.

Console: Advanced JavaScript error console for Firefox.

JS Eclipse: A JavaScript plugin for the Eclipse IDE.