For college algebra and precalculus:

http://www.stitz-zeager.com

For calc 1 - 3:

http://www.apexcalculus.com

I did find one really good intermediate algebra book but can’t find the website.

University of Wisconsin Calculus 1/2/3 textbooks, GNU public licensed.

https://github.com/fabiohenriquecn/wisconsin-calculus

edit: I guess as of this semester these are no longer being used in classes, they've moved to the online homework / $300 textbook model.

The online homework tied to the book serial number is especially insidious.

It's only ~60$

List by a Math professor: http://people.math.gatech.edu/%7Ecain/textbooks/onlinebooks....

Book of Proof by Richard Hammack:

https://www.people.vcu.edu/~rhammack/BookOfProof/