Note that these cards are also available in physical format, which you can purchase at Printer Studio.

Columbia University's Asia for Educators provides resources relating to Asian history, art, and geography

National Geographic Education
https://www.nationalgeographic.org/education

Best of History Web Sites is " an award-winning portal created for students, history educators, and general history enthusiasts"; sites are rated for usefulness and accuracy
https://besthistorysites.net/

History Matters: "the U.S. survey course on the web". Designed for high school and college teachers of U.S. history
http://historymatters.gmu.edu/

BrainU offers lesson plans, videos, teacher resources for K-12

Backyard Brains has "exciting and affordable entry‐level Brain Recording Kits that provide the ability for students of all ages to learn about neurons"

Flipping Physics has videos for high school physics teachers to "flip" the classroom

The Physics Classroom has a lot of great resources for high school physics, including Physics Interactives, Teacher Toolkits, and Concept Builders

Concord's High School Biology Resources provides visual simulations for teaching science to high school students

The Missouri Botanical Garden has quite a nice site on biomes and ecosystems
http://mbgnet.mobot.org/

Understanding evolution is a website for teachers, aimed at explaining evolution, pointing out relevance, misconceptions, pitfalls, and so on.
http://evolution.berkeley.edu/

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.

NASA has a nice Solar System Exploration website

ASPIRE - Astrophysics interactive labs for teachers
http://aspire.cosmic-ray.org/

Spacelink: instructional materials from NASA
http://spacelink.nasa.gov/Instructional.Materials/

NASA Spitzer Space Telescope
http://www.spitzer.caltech.edu/

Multiverse is "A center of multicultural science education to support diversity in earth & space science."

The American Geosciences Institute have an array of curriculum materials for all levels, from elementary to college.