1. Khan Academy

Subjects: Math, Science, Economics, History, and more.
Rather an extensive library of video lessons and exercises in most subjects. Free and available to anyone of any age.

2. Coursera

Subjects: Business, Data Science, Computer Science, Arts, Humanities, and more.
Partners with top universities and organizations to offer online courses, certificates, and degrees.

3. edX

Subjects: Business, Health, Education, Engineering, Computer Science, etc.
Free, high-quality courses from top universities like MIT, Harvard, and Berkeley.

4. Udacity

Subjects: Technology-related courses in AI, Data Science, Programming, and so on.
Recognized by its well-known “nanodegree” programs that closely prepare its users for careers in the technology field.

5. Duolingo

Subject: Language learning.
A widely used app and website that teaches languages through gamified lessons.

6. MIT Open Courseware (OCW)

Subjects: Engineering, Computer Science, Humanities, Sciences.
Has MIT course materials available online free of charge.

7. Skillshare

Subjects: Creative skills such as Photography, Design, Writing, and more.
It operates on a membership basis, having project-based learning.

8. FutureLearn

Subjects range upon Health; History; Technology; Business, and many more.
This platform also provides courses and programs in partnership with universities and institutions in short formats.

9. National Geographic Kids

Subjects: Science, Geography, History, Animals, and so on.
It contains educational games, videos, and articles that appeal to younger audiences.

10. BBC Bitesize

Subjects: Core subjects for primary and secondary school levels.
Interactive lessons, videos, and quizzes for UK curriculum.

11. TED-Ed

Subjects: Diverse topics in Science, Arts, Philosophy, among many others.
TED Talks and TED-Ed videos on curiosity sparking and learning.

12. LinkedIn Learning

Subjects: Business, Technology, Creative professional skills. Subscription-based platform offering courses taught by industry experts.

13. OpenLearn (Open University)

Subjects: History, Health, Business, Psychology, plus more. Free courses and course materials from the UK’s Open University.

14. Codecademy

Subject: Coding, programming languages including Python, JavaScript, HTML. Interactive website to learn how to code.

15. NASA for Students

Topics: Astronomy, Earth Science, and STEM.
Informational resources, videos, and activities for students on the site.

16. Brilliant.org

Topics: Mathematics, Science, Engineering, and Computer Science.
Incorporates interactive problem-solving along with critical thinking.

17. Quizlet

Topics: Flashcards and study tools for a large number of subjects.
Students develop their memorization and build readiness for exams through interactive learning.

18. Academic Earth

Topics: Business, Engineering, Science, Humanities, and more.
This site aggregates free online courses from several universities.

19. Scholastic

Subjects: Reading, Science, Math, and Social Studies for kids.
Offers teaching resources, student and parent resources, games and activities.

20. Alison

Subjects: Health, IT, Business, Languages, and Personal Development.
Free online courses with a very strong focus on work related skills / certifications.