Khan Academy: https://www.khanacademy.org/
With practice exercises, instructional videos, and a personalized learning dashboard, Khan Academy empowers learners to study at their own pace, both in and out of the classroom. Covering subjects from kindergarten through early college, including math, science, reading, computing, history, art history, economics, financial literacy, SAT, MCAT, and more, Khan Academy focuses on skill mastery to help learners build the strong foundations they need to succeed in higher education and in their careers.
Teacher Tube: https://www.teachertube.com/
Our goal at TeacherTube has been to provide an online community for sharing instructional and education videos for college students, high school students, middle school students and elementary school students. We seek to fill a need for a more educationally focused, safe venue for teachers, students, schools, and home learners. It is a site to provide anytime, anywhere professional development with teachers teaching teachers. It is also a site where teachers can post videos designed for students to view in order to learn a concept or skill.
Course Hero: https://www.coursehero.com
Instant access to millions of Study Resources, Course Notes, Test Prep, 24/7 Homework Help, Tutors, and more. Learn, teach, and study with Course Hero.
Spark Notes: https://www.sparknotes.com/
SparkNotes is a resource you can turn to when you're confuzzled. We help you understand books, write papers, and study for tests. We're clear and concise, but we never leave out important info.
As SparkNotes editors, our mission is to help you make sense of confusing schoolwork. We are well qualified to lend a hand: we're graduates of top schools, we have advanced degrees galore, we've taught undergraduate and graduate classes, and we've edited books on Shakespeare, The Scarlet Letter, and the SAT (and that's just the S's!). We work with experts to create books, blogs, quizzes, and flashcards that will help you master hard material.
CK-12: https://www.ck12.org/student/
CK-12 will provide content and technology tools to help teachers provide learning for students in a global world. Free access to high-quality, customizable educational content in multiple modalities suited to multiple student learning styles, levels, resources and use content acceptance will allow teachers, students and others to innovate and experiment with new models and modalities of learning. CK-12 help students and teachers alike by enabling rapid customization and experimentation of teaching and learning styles.