Free Math Resources

Bookboon has thousands of free ebooks, including math books.

Openstax

Physics Forums has a list of free math books.

Open Textbook Library

Hamsphire College

Open Library has free math books.

CK-12 also has free math books.

Free and Legal Math Books.

Lumen Learning

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.

OpenLearn

Pual's Online Notes

Wikibooks free books that anyone can edit.

Wikiversity free course materials.

Wikipedia has some articles about math.

Wikipedia Math Books.

Saylor

Wikihow has some math articles.

Mathispower4u has lots of videos.

PatrickJMT also has videos.

OpenTextbookStore

MyOpenMath

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

Washington 45

BCcampus

OpenIntro

this Medium article has links to 951+ math books.

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

TextbookRevolution

Math Forum Library

Open SUNY has a few math books.

Open Textbooks

Free science

LibreTexts

Awesome math books, lecture notes, videos, etc.

MathTV has a lot of videos from multiple instructors.

Alcumus is good for practicing.

AoPS videos

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

Siyavula

Lyryx

Kirkwood

GALILEO

APEX Calculus

APEX PreCalculus

SpringerOpen

Mathematics Books Online

Hong Kong Open Textbooks

Open Up Resources

Open Mathbook

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

Probability.net

Expii

e-Calculus

Game Theory.net

Mathematics Archive

Mathematics Center

CUNY Academic Works

CliffsNotes

SparkNotes

Union Test Prep

Wolfram MathWorld

Wyzant Resources

Math.com

School Yourself

IXL

17Calculus

GeoGebra

Open Course Library

Smashwords

UCI Open

Virtual University of Pakistan OCW

Michel van Biezen

xactly.com

Wolfram alpha problem generator

Curriki

Big Ideas Math Free Easy Access.

Maphi is an app for practicing Mathematics.

Microsoft Math Solver

ProfRobBob

Professor Leonard

Hippocampus

WebMath

Digital Commons Network

Grasple

RealNotComplex

eCampusOntario

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/