JavaScript resources
General
mozilla developer center - Firefox, CSS, HTML, JS, etc.
SitePoint - CSS, HTML, JS, compatibility.
w3schools - CSS, HTML, JS, compatibility, etc.
Developer tools
Firefox: FireBug and its extensions.
Firefox: Web Developer.
IE: Developer Tools - built-in.
Opera: DragonFly - built-in.
WebKit (Chrome, Safari, etc.): Web Inspector - built-in.
Libraries
Canvas 3D (C3DL) - Interactive 3D, requires an extension, Firefox only.
flot - Interactive 2D plotting.
JavaScript InfoVis Toolkit - 2D visualization.
jQuery - Set up, find, and modify attributes, CSS, DOM, effects, events, AJAX, etc.
jQuery UI - Themable widgets and effects.
jsMath - Typeset mathematics.
O3D Interactive 3D, requires a plug-in.
Processing.js - Interactive applications.
Protovis - 2D visualization.