1. Khan Academy: Offers a wide range of educational content across various subjects, including mathematics, science, and humanities.

2. EdX: Provides access to university-level courses from leading institutions worldwide, allowing teachers to expand their knowledge and skills.

3. TED-Ed: Features engaging educational videos and interactive lessons on various topics, making learning more enjoyable for both teachers and students.

4. National Geographic Education: Offers free resources, such as lesson plans, activities, and multimedia materials, to enhance geography and science education.

5. Smithsonian Learning Lab: Provides a vast collection of digital resources, including images, videos, and interactive tools, to support inquiry-based learning.

6. Common Sense Education: Offers reviews and ratings of educational apps, games, and websites, helping teachers find high-quality digital resources for their classrooms.

7. Open Culture: Curates a comprehensive list of free online courses, e-books, and educational resources, ranging from literature to music.

8. PBS LearningMedia: Features thousands of digital resources aligned with national standards, allowing teachers to create engaging lesson plans and activities.

9. Google for Education: Provides free tools, such as Google Classroom and G Suite for Education, to enhance collaboration and streamline classroom management.

10. TeachThought: Offers a wide range of teaching strategies, lesson plans, and resources
aimed at fostering critical thinking and creativity in the classroom.

11. ReadWriteThink: Provides free lesson plans, interactive activities, and professional development resources for English Language Arts instruction.

12. Big History Project: Offers a free, interdisciplinary course that explores the history of the universe, from the Big Bang to the present day.

13. Open Education Resources Commons: Provides a vast repository of freely available educational materials, including textbooks, videos, and lesson plans.

14. CK-12 Foundation: Offers customizable educational content, including textbooks, interactive simulations, and study guides, across various subjects.

15. National Science Foundation: Features a collection of resources, videos, and lesson plans to support STEM (Science, Technology, Engineering, and Math) education.

16. Free Rice: Offers free educational games that help students improve their knowledge in subjects like vocabulary, math, and science while also donating rice to fight hunger worldwide.

17. NASA for Educators: Provides a range of resources, activities, and lesson plans to engage students in space science and exploration.

18. Scholastic Teachables: Offers a subscription-based service providing access to thousands of downloadable educational resources, including lesson plans, worksheets, and activities.

19. Library of Congress: Provides free access to primary source materials, lesson plans, and professional development resources to enhance history and social studies instruction.

20. OpenStax: Offers free, peer-reviewed textbooks and resources for college and high school courses, covering various subjects.
These resources, ranging from websites to online courses, can greatly assist teachers in providing quality education and engaging classroom experiences for their students, all for free.