cK-12
This site provides full, customizable ‘FlexBooks’ and interactives for a wide range of ages and topics. Most of the materials are for STEM fields, but there are also resources for English and History.
Grades K-12

HippoCampus
“Free educational resources for middle school to college.” Over 6,800 free videos for learning math, sciences, social sciences, and the humanities.
Grades 6-12

MIT OpenCourseWare
“MIT OpenCourseWare (OCW) is a web-based publication of virtually all MIT course content. OCW is open and available to the world.”
Grades 9-12

OER Commons
OER is shorthand for Open Educational Resources – free resources that teachers are allowed to copy and adapt for their own purposes. The OER commons includes textbooks and entire courses, as well as individual lessons and activities.
Grades K-12 (primarily 9-12)