Open Access Books
Rice Connexions is providing peer reviewed, quality open textbooks with interactive components. There are some amazing textbooks available here, with more in production.
BC OpenEd
A curated collection of open textbooks, many reviewed by British Columbia faculty.
Project Gutenberg
Find the full text of classics and public domain works from the first massive ebook creating organization in existence. Nothing fancy here, just files with the full text.
American Institute of Mathematics
A list of open textbooks in various subdisciplines of Mathematics approved by the AIM editorial board.
FreeBooks4Doctors
Free medical textbooks.
Bloomsbury Academic
Bloomsbury is a well-respected and long time UK publisher who has released some of their academic titles for open access/open education.
Hundreds of complete, open college-level textbooks collected by the University of Minnesota.
WikiBooks
A project of the Wikimedia Foundation, this collection of group written textbooks in a variety of sources follows rules similar to Wikipedia.
Google Books
Some books presented in this mass conglomeration of scanned books are fully available, most are excerpted.
Aviation Handbooks and Manuals
Open Textbooks on Aviation from the Federal Aviation Administration (US).
Lyryx Open Textbooks
Lyryx offers high-quality open textbooks in the fields of accounting, mathematics, and economics. They also have optional added (paid) resources such as homeworks and quizzes to accompany the open textbooks.