Khan Academy

Everything is learnable. Always free.
Spend an afternoon reviewing shapes and geometry. Explore the stars and black holes in the universe. Get to know various investment tools and learn financial knowledge. Understand English grammar and style.
Whether you're a student, teacher, self-learner, principal, an adult who wants to go back to the classroom 20 years later, or a friendly alien just trying to fit in on Earth and study biology - Khan Academy's resources are completely open and free for you to use.
- Learn anything for free: Thousands of interactive exercises, videos, and articles are at your fingertips. Learn math, science, economics, finance, grammar, and other subjects. (More subjects are being translated!)
- Improve your skills: All exercises come with instant feedback and step-by-step hints. Follow your school curriculum or practice at your own pace.
- Keep learning offline: Bookmark and download your favorite content to watch videos without an internet connection.
- Continue your online learning: Your learning progress is automatically synced with zh.khanacademy.org. So your progress is always up-to-date, anywhere, anytime.
Learn math (grades 1 - 6 of the People's Education Edition), science (cosmology, medicine), economics, grammar, and more by watching videos, doing interactive exercises, or reading articles!
Khan Academy is a U.S. 501(c)(3) non-profit organization. Our mission is to provide free, world-class education for anyone.