Note that these cards are also available in physical format, which you can purchase at Printer Studio.
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
History Matters: "the U.S. survey course on the web". Designed for high school and college teachers of U.S. history
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
Understanding evolution is a website for teachers, aimed at explaining evolution, pointing out relevance, misconceptions, pitfalls, and so on.
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
Spacelink: instructional materials from NASA
NASA Spitzer Space Telescope