JavaScript resources
References
Browser compatibility - QuirksMode, Web Devout, When can I use....
Cheat sheets, etc. - Canvas, jQuery API browser.
JSON.
mozilla developer center - AJAX, CSS, DOM, HTML, JS, XML, etc.
Unicode - categories / index / search, scripts and symbols, UTF-8 tool.
Validators - CSS, HTML / MathML / SVG / XML, Link Checker, JSLint.
World Wide Web Consortium / W3C - HTML5, MathML, SVG, etc.
Tutorials
Demos and examples (see Libraries, too)
Libraries
CAKE - 2D scenegraphs.
Canvas 3D / C3DL - Requires an extension, Firefox only.
Explorer Canvas - Canvas for IE.
flot - 2D plotting.
Jmol - 3D visualization, requires Java.
jQuery - Set up, find, and modify attributes, CSS, DOM, effects, events, AJAX, etc.
jQuery SVG - 2D plotting, scenes.
jQuery UI - Themable widgets and effects.
jsMath - Typeset mathematics.
jsTree - Tree widget.
JSXGraph - 2D interactive geometry, plotting, visualization.
O3D - Requires a plug-in.
Pre3d - Visualization.
Processing.js - Interactive applications.
Protovis - 2D.
pyjamas - Python to JS compilation, AJAX, widgets.
Raphaƫl - SVG / VML vector graphics.
SVG-edit - Inkscape-lite.
svgweb - SVG natively or via Flash.
Developer tools
Firefox - FireBug and its extensions, Web Developer.
IE - Developer Tools - Built-in.
Opera - DragonFly - Built-in.
WebKit (Chrome, Safari, etc.) - Web Inspector - Built-in.