Textbooks:
Boundless Textbooks (College)
Saylor University (College)
CK-12 (K-12)
Course Supplements (Distance Learning):
Saddleback College Algebra
MIT Offline
Study.com CLEP
Self-paced ABE equivalent:
KA-Lite (K-12 Interactive)
Math Expression
Understanding Algebra (James Brennan)
PhET Science Simulations
Reference:
Wikisource
Oregon Law Library
Wikipedia for Schools
Wiktionary
Life Skills:
Fairshake Re-entry Resources
Fantastic Phonics Literacy
GCF Learn Free
Robotics
Farming and Agriculture
Music Theory
Storybooks and Web Design
Entertainment:
Great Books of the World
Radiolab
TED Talks
Wikivoyage