Bookboon has thousands of free ebooks, including math books.
Physics Forums has a list of free math books.
Open Textbook Library
Open Library has free math books.
CK-12 also has free math books.
Free and Legal Math Books.
Lumen Learning Boundless Courses
Open Culture is awesome! it has links to online courses and books.
MIT Online Textbooks
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.
Pual's Online Notes
Wikibooks free books that anyone can edit.
Wikiversity free course materials.
Wikipedia has some articles about math.
Wikipedia Math Books.
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/
Online Mathematics Textbooks
this Medium article has links to 951+ math books.
The Global Text Project website is down, but links on wayback machine work.
Math Forum Library
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.
Mason OER Metafinder
Mathematics Books Online
Hong Kong Open Textbooks
Open Up Resources
Free Textbooks List
Open Access Texts
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
CUNY Academic Works
Union Test Prep
Open Course Library
Virtual University of Pakistan OCW
Michel van Biezen
Wolfram alpha problem generator
Big Ideas Math Free Easy Access.
Maphi is an app for practicing Mathematics.
Microsoft Math Solver
Digital Commons Network
Edfinity Problem Repository
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/