CK-12
Learn everything from 1st Grade Math to PreCalculus. Another excellent alternative to Khan Academy if you are looking for a slightly different style.
The CK-12 Foundation is a California-based non-profit organization which aims to increase access to low-cost K-12 education in the United States and abroad. CK-12 provides free and customizable K-12 open educational resources aligned to state curriculum standards.
By the numbers:
- 255 million+ users served
- 2.33 billion+ answers provided
- 44 million+ hours of learning available on the site