BCcampus-OpenEd - a collection of more than 150 online textbooks, the result of Canada's first government-funded open textbook project. Subjects include arts, business, health-related sciences, and trades (culinary arts).
BookBoon - free textbooks can be downloaded. Subjects include IT & programming, accounting, natural sciences, marketing & law, and engineering. Older versions of Microsoft Office are also available.
Curriki - a hub for free, open course materials. Find thousands of course materials in a variety of subjects and grade levels, including free courses and textbooks.
Directory of Open Access Books (DOAB) - this website includes a large selection of international textbooks covering subjects such as business and economics, languages and literatures, mathematics and statistics, technology and engineering, and many more.
Flat World Knowledge - a college textbook publisher whose books are published under an open license. This allows professors to customize the books they order – edit, add to, mix-up – or use as-is. Students can access the books online for free or can pay for print-on-demand and audiobook versions.
InTechOpen - the world's largest science, technology, and medicine open access book publisher, with nearly 3,000 books and more than 95 million downloads.
MERLOT - Multimedia Educational Resources for Learning and Online Teaching (MERLOT) is a project of California State University and offers peer reviewed online teaching and learning materials.
The National Academies Press - the NAP publishes 200+ books a year on a wide range of topics in science, engineering, and medicine, providing authoritative information on important matters in science and health policy.
OAPEN - a quality controlled collection of open access books, mainly in the area of Humanities and Social Sciences.
OER Commons Open Textbook Hub - a platform on which educators can create or remix open educational resources for lessons and modules. An extensive library includes lesson plans, video/audio lectures, syllabi, textbooks, simulations, and full courses. A section specifically for STEM-related materials is linked to the home page for easy access.
Open Access Textbooks Websites - a FIPSE grant project, creating a model for open textbook implementation. This is their list of open textbooks sites.