Recommended for Grades K+. Free website.
Phet provides interactive simulations for math, physics, earth science, biology, and chemistry. Search by topic, grade level, and device.
Recommended for Grades 1-6. $3.99/app.
TinyBop offers different apps that allow kids to explore the invisible wonders of the world. Explore how the human body works, and learn about plants, earth, and space through this app.