Initiated by the California State University system, the Multimedia Educational Learning Resource for Learning and Online Teaching (MERLOT) is a database that includes open textbooks among its searchable resources.
This program, started by Rice University in 2012, has created more than twenty open online textbooks written and peer reviewed by educators, with an emphasis on creating textbooks for courses that traditionally have a high number of enrolled students.
Bookboon, a textbook company situated in Denmark, provides access to approximately 200 open online textbooks dealing with various topics. Note that not all books available on all areas of the website are free; some are "premium" books available only for a price.
OER Commons, created by nonprofit Institute for the Study of Knowledge Management in Education (ISKME), is a hub providing access to open textbooks and other educational materials across different academic fields.
The Orange Grove is a Florida-based statewide institutional repository for open educational materials, including textbooks. Note that some of the textbooks labeled "K-12" actually deal with college-level topics.