*There is some controversy over the quality and accuracy of the information in the tutorials offered by W3Schools. Not as many, however, take issue with the reference section.
Related questions, mostly the source of the above links:
I find the Mozilla Developer Network very helpful.
The Unofficial online ECMAScript-262 3rd ed. specification and Annotated ECMAScript 5.1 are very helpful.
A really helpfull tool for Firefox (with Firebug installed) http://tools.sitepoint.com/codeburner/
For rapidly find documentation on sitepoint reference.
While not directly a reference site, LearningLine rolls many of these sites into their lessons as well as sites like http://caniuse.com/.
Here are some of the courses that are good starting points:
I've found AeroTwist very useful for learning rendering performance.
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