OpenIntro

https://www.openintro.org
OpenIntro's mission is to make educational products that are free, transparent, and lower barriers to education.
OpenIntro is a 501(c)(3) US-based nonprofit that is independently operated. We currently offer or support over half a dozen free textbooks that are also available as low-cost paperbacks. While we are not part of any college or university, we frequently partner with faculty to develop and test resources.