Visionlearning provides "educational content in science that is grounded in best practices of how people learn while still being concise and freely available" Their materials are targeted to both students and educators, and available in both English and Spanish. You can also choose to have them written in OpenDyslexic font.
Carleton College’s Science Education Research Center has produced a resource on "Guided Discovery Problems" and how they can help teach science, together with some examples.
The University of California Museum of Paleontology has developed a website to help teachers get young people interested in science. Understanding Science aims to be “a fun, accessible, and free resource that accurately communicates what science is and how it really works."
National Science Digital Library has resources for science, technology, engineering, and mathematics education
BioInformatics Free Resources for Science Education
Assessment resources in Math, English, Science relating to the New Zealand curriculum
Middle School Science has Free Lesson Plans & Resources for grades 5 – 8
Frank Potter's Science Gems apparently has over 14,000 science resources sorted by category, subcategory, and grade level.
The Khan Academy is chock full of instructional videos, mainly on math and science topics. https://www.khanacademy.org/
Worseley School Science Files: science and math resources for all grades from middle elementary through senior high school
The American Association for the Advancement of Science site presents 600 items that teachers can use to test middle and early high school students' understanding in the earth, life, physical sciences, and the nature of science. Importantly, it also tests for common misconceptions. http://assessment.aaas.org/
Here's one I want to urge on any science teachers, believing as I do that every citizen should understand the basis of scientific thinking (and particularly the scientific meaning of the word 'theory'!), something that apparently so few (and so few teachers) do. Peer Review Education Resource provides several lesson plans to help teachers understand and explain how real science works.
Teachers might find the Art Institute of Chicago's course exploring the relationship between science and art useful and inspirational.
Super Science Fair Projects for help and inspiration with your science fair
The Exploratium has a collection of websites, apps, etc, relating to science:
Howtoons are one-page cartoons showing 5-to-15 year-old kids "How To" build things. Each illustrated episode is a stand-alone fun adventure accessible to all, including the pre-literate.
TeachEngineering has "teacher-tested, standards-based engineering content for K-12 teachers to use in science and math classrooms" at https://www.teachengineering.org/