MDN is a developer reference wiki that offers educational tracks for learning different web technologies. Run by a non-profit organization, all information on the site is user verified and editable. The site offers different level tutorials based on your skill level, ranging from introductory to advanced, as well as links to external sites that could be of use, such as StackOverflow and TogetherJS.
The wiki based website allows for information to be quickly added or changed as mistakes are noticed and new technologies arise. The sleek website is easy on the eyes and easy to navigate making it an ideal reference for anyone trying to learn web-based languages and tools.