JavaScript Reference

Mozilla Developer Center

This site has a comprehensive JavaScript reference, a guide to programming with JavaScript, links to helpful tools, and more:

http://developer.mozilla.org/en/docs/JavaScript/

Dev.Opera

While focused primarily on its own browser platform, Opera's site for web developers includes a number of useful articles on JavaScript:

http://dev.opera.com/articles/

Quirksmode

Peter-Paul Koch's Quirksmode site is a terrific resource for understanding differences in the way browsers implement various JavaScript functions, as well as many CSS properties:

http://www.quirksmode.org/

JavaScript Toolbox

Matt Kruse's JavaScript Toolbox offers a large assortment of homespun JavaScript libraries, as well as sound advice on JavaScript best practices and a collection of vetted JavaScript resources elsewhere on the Web:

http://www.javascripttoolbox.com/