MDN - The Mozilla Developer Network
W3Schools have both refs and tutorials
HTML5 Please - Look up HTML5, CSS3, etc features, know if they are ready for use, and if so find out how you should use them – with polyfills, fallbacks or as they are.
OverAPI.com - Collecting all the cheat sheets
RealWorld - "The mother of all demo apps"
Regulex - Displays your regex as a railroad diagram