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These are the best Computer Science websites I've discovered over the years.

Stanford Engineering Everywhere - free, high quality Computer Science education from Stanford. Includes several AI courses and electrical engineering courses.

MIT OpenCourseWare - a huge collection of Computer Science courses from MIT. Includes "Introduction to Computer Science and Programming", "Introduction to Computational Thinking and Data Science", "Structure and Interpretation of Computer Programs" and much more.

W3Schools - some of the best courses for students who want to learn AJAX, SQL, CSS, JavaScript, html, etc.

Academic Earth - 83 Computer Science courses, including "Affective Computing", "An Introduction to Computer Networks", "Atomistic Computer Modeling of Materials", "Building Dynamic Websites", etc.

Harvard Extension School - Harvard will offer some of its courses for free every now and then. Their most recent offer is "Introduction to the intellectual enterprises of computer science and the art of programming". It teaches students algorithms, data structures, software engineering, web development, and more, using the C, Python, SQL and JavaScript programming languages.

Wikibooks - the best solution for people who prefer to learn one of the Computer Science disciplines by reading a book. Includes hundreds of well written, easy to understand books: "Computers for Beginners", "Computer Animation", "Guide to Game Development", "Non-nerd's Guide to Computers", "Computer Graphics", "High performance computing", "Data Science: An Introduction", etc.

Codecademy - a website that helps you learn programming online, interactively, without having to install an IDE on your computer.

Sofia, aka Sharing Of Free Intellectual Resources, is a website that offers free courses for students who want to learn "Creative Typography", "Elementary Statistics", "Enterprise Network Security", "Introduction to Java Programming", "Introduction to Macromedia Flash" and "Webpage Authoring".

Carnegie Mellon OpenLearning Initiative - this prestigious university offers 30 free courses. Be sure to check out "Causal & Statistical Reasoning", "Argument Diagramming", "Engineering Statics", "Health Information Technology Foundations", "Logic & Proofs", "Media Programming", "PC Hardware: Cyber Pathways Across Maryland", "PC Software: Cyber Pathways Across Maryland" and "Principles of Computing with Python".

Khan Academy - The last on the list, but among the best out there! Be sure to check out "Computer programming", "Computer science" and (most of all) "Hour of Code", which helps you learn the basics of programming, website building or database creation and management in only one hour.