CK-12 Foundation provides a library of free online textbooks, videos, exercises, flashcards, and real world applications for over 5000 concepts from arithmetic to History
Khan Academy is a non-profit educational organization created in 2008 by Salman Khan with the goal of creating a set of online tools that help educate students. The organization produces short lessons in the form of videos. Its website also includes supplementary practice exercises and materials for educators
WatchKnowLearn organizes thousands of free educational video clips in all subjects for teachers, students, and parents.
TED Talks are influential videos from expert speakers on education, business, science, tech and creativity, with subtitles in 100+ languages.