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Wiki books
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Wikiversity
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WikiEducator - dedicated to building a complete education curriculum by 2015
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ck12 - free, flexible textbooks for the whole American educational syllabus
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Disqo
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Khan Academy - a series of tutorial videos, mostly of science and maths, explaining concepts very well.
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MIT's YouTube channel - Includes entire lecture series. The one on linear algebra in damn good.
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Open Yale Courses
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Stanford's YouTube channel
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United Nations University OpenCourseWare
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John Hopkins OpenCourseWare
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eToys - free and open-source software that teaches ideas in innovative ways. For kids
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Open University's Open Learn
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P2P university]
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Connexions - Open educational content in hundreds of subjects.
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List of educational video websites on Wikipedia
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Open Culture - list online courses
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Wolfram Demonstrations Project - A compilation of pretty interactive visualizations of hundreds of phenomena in science, mathematics, music and other areas.
Enjoy!