1. Teachers Pay Teachers: A platform where educators share and sell their resources, including many free options.

2. Khan Academy: Offers a wide range of educational content and resources for teachers and students.

3. National Geographic Education: Provides free lesson plans, activities, and resources covering various subjects.

4. Smithsonian Learning Lab: Access thousands of resources from the Smithsonian Institution on various topics.

5. TED-Ed: Find educational videos, lessons, and resources designed to spark curiosity and learning.

6. PBS LearningMedia: Offers free access to high-quality educational content from PBS and its partners.

7. ReadWriteThink: Provides lesson plans, activities, and resources for teaching reading and writing skills.

8. Library of Congress: Explore digital collections, primary sources, and teaching resources from the Library of Congress.

9. CommonLit: Access a library of free reading materials and accompanying lesson plans for grades 3-12.

10. NASA for Educators: Discover educational resources related to space, science, and engineering from NASA.

11. Google for Education: Explore free tools, resources, and lesson plans for using technology in the classroom.

12. Edutopia: Find articles, videos, and resources focused on innovative teaching practices.

13. Open Culture: Offers a curated collection of free educational resources, including textbooks, audiobooks, and courses.

14. PhET Interactive Simulations: Access online interactive simulations for science and math education.

15. National Science Teaching Association: Provides free resources, articles, and activities for science educators.

16. Free Technology for Teachers: Discover free technology tools and resources that can enhance teaching and learning.

17. History Matters: Access resources for teaching history, including primary sources and interactive exhibits.

18. LearnZillion: Provides free and premium resources for math and English language arts instruction.

19. Science Buddies: Offers free resources and project ideas for hands-on science experiments.

20. TeachHUB: Find lesson plans, tips, and resources for teachers across various subjects.

These are just a few of the many excellent sources of free teacher resources available.