Openstax College:
http://openstaxcollege.org/books
Openstax CNX:
http://cnx.org/
Open Textbook Library:
http://open.umn.edu/opentextbooks/
College Open Textbooks:
http://www.collegeopentextbooks.org/
Saylor Foundation:
http://www.saylor.org/books/
Open Culture:
http://www.openculture.com/free_textbooks
Bookboon.com:
http://www.bookboon.com
The Global Text Project:
http://globaltext.terry.uga.edu/books
BC Campus:
https://open.bccampus.ca/find-open-textbooks/
California Open Library for Education:
http://www.cool4ed.org/
Free Book Centre:
http://www.freebookcentre.net/
InTech:
https://www.intechopen.com/
Open SUNY Textbooks:
https://textbooks.opensuny.org/
Wikibooks:
https://en.wikibooks.org/wiki/Main_Page
Textbook Revolution:
http://textbookrevolution.org/index.php/Book:Lists/Subjects