OpenStax College

Initiated by Rice University, their free textbooks are developed and peer-reviewed to ensure they meet the scope and sequence requirements of college courses.
  Saylor Academy
Collection of freely available open textbooks for download, online reading, and sharing.
The LibreTexts mission is to unite students, faculty and scholars in a cooperative effort to develop an easy-to-use online platform for the construction, customization, and dissemination of open educational resources (OER) to reduce the burdens of unreasonable textbook costs to our students and society.
  Directory of Open Access Books
The Directory of Open Access Books (DOAB) is a discovery service for Open Access monographs.
  The University of Minnesota Open Textbook Catalog
This site is a clearinghouse of links to books housed in various locations, including OpenStax, Saylor and others, with over 140 books.
  College Open Textbooks
Focused on driving awareness and adoptions of open and peer-reviewed textbooks to more than 2,000 community colleges.
  BC Campus OpenEd
This collection includes texts written specifically for the BC OpenTextbook initiative. 

  Open SUNNY Textbooks
With the initiative of the SUNY Libraries, Open SUNY Textbooks publishes high-quality, peer-reviewed textbooks for use in higher education.
  The National Academies Press
Unlike some of the open textbook initiatives these books are publicly available but not openly licensed.  You can link to the content, and even link directly to specific pages.  However, you cannot remix and redistribute the content.
  Textbook Equity
Creator, publisher, distributor, and seller of open textbooks and ancillary materials to college-level students.
  Global Text Project
Joint venture between the University of Georgia and University of Denver business schools. Provides a free library of thousands of texts on various topics.
European eBook publisher with 1000+ textbooks freely available to download. E-books are primarily in business/economics, engineering/natural sciences and IT.
  Open Textbooks for Hong Kong
The Open Textbooks for Hong Kong project aims to establish a sustainable system which provides quality open textbooks for adoption and adaptation at minimal cost for education for all.