Bookboon has thousands of free ebooks, including math books.

Physics Forums has a list of free math books.

Open Library has free math books.

CK-12 also has free math books.

Lumen Learning Boundless Courses

Open Culture is awesome! it has links to online courses and books.

MIT OpenCourseWare lecture notes, videos, resources, etc.

Mathplanet has lessons for pre-algebra, algebra I, algebra II, etc.

Gutenberg has some math books.

archive.org also has math books.

Open Textbook Initiative has links to approved free textbooks.

Wikibooks free books that anyone can edit.

Wikiversity free course materials.

Wikipedia has some articles about math.

Wikihow has some math articles.

Mathispower4u has lots of videos.

PatrickJMT also has videos.

pre-algebra was on open textbook store but link was dead so here's a working link.

OER Commons is a search engine.

MERLOT is also a search engine.

A reddit post has free math resources with emphasis on Open Source Textbook.

PBS LearningMedia classroom resources search (for teachers ?).

you can find free books by searching :*subject* open source book

e.g. algebra open source book

on google or duckduckgo

on google you can also use advanced search and select usage right.

for finding PDFs you can also use http://www.pdfsearchengine.net/ or http://www.pdfsearchengine.info/

this Medium article has links to 951+ math books.

The Global Text Project website is down, but links on wayback machine work.

Open SUNY has a few math books.

Awesome math books, lecture notes, videos, etc.

MathTV has a lot of videos from multiple instructors.

Alcumus is good for practicing.

Crash Course has statistics videos.

Purplemath has a lot of written Math tutorials.

Guide on How to find Open Textbooks.

OASIS is a Search engine.

History of Mathematics Archive

You can search Pressbook books on Google like this:

"powered by pressbooks" intitle:<Search Term> -inurl:chapter -inurl:front-matter -inurl:back-matter

Virtual University of Pakistan OCW

Wolfram alpha problem generator

Big Ideas Math Free Easy Access.

Maphi is an app for practicing Mathematics.

NumeradeTextbook solutions (including Open Textbooks).

https://www.getfreeebooks.com/ https://www.freetechbooks.com/ https://oer-obp.pubpub.org/pub/wac0y6kx/release/9 https://instr.iastate.libguides.com/oer/ http://www.zillman.us/subject-tracers/education-resources/ http://www.oersources.com/ http://www.2020directoryofdirectories.com/ https://goopenva.org/ https://pdfhost.io/