This list will change often.
Mozilla Developer Network
Sitepoint articles are always informative and well-written. It surprises me how actively this site is updated.
If interactive instruction is your thing, then Codecademy is a great place to dive-in. The interface is clean and the content is quite good.
The coverage is somewhat brief, but overall well-done. The author is Russian and gave-up on the English version of this site because of his frustration with English. There is a link to the Russian version of the site, which offers a great deal of content and is actively updated. If you use Google Chrome, then you can translate these pages on the fly.