Open Educational Resources
BC Campus Open Ed Open Textbook Database
CK12
Community College Consortium for Open Educational Resources
freeCodeCamp
Personal comment:
It has a forum to interact and has its curriculum open sourced.
IXL
JHU Data Science Lab
Khan Academy
Personal comment:
It's well known and has I think one of the biggest learners' community.
Math Wiki
Personal comment:
Well, it is structured like a textbook and it is said that the Math Wiki is a textbook so there's that.
Merlot
MIT OpenCourseWare
Personal comment:
Mah boi, I recommend this by a loooooong shot.
MIT Press Open
OER Commons
Open Library
Open SUNY Textbooks
Open Textbook Library
Personal comment:
If you want to look for a wide database of open textbooks, here's one of them that I personally use.
Open University
OpenStax CNX
OpenStax
Personal comment:
I recommend this one a lot. It has attained large backup from foundations such as Melinda and Gates Foundation, Google, The Open Society Foundation, and the National Science Foundation.
OSDev.org
ossu
Teach Yourself CS
The Feynman's Lectures on Physics
The Open Source Computer Science Degree
Wikibooks
WikiToLearn
Personal comment:
One of my personal recommendations right here.
Personal comment:
Recommended for looking/reviewing into high school sciences curriculum.