Saylor is a site that creates courses, some of which have open textbooks. There are over 300 courses online and they are mainly for college-level education. The goal is to create classes by combining a number of resources to create a holistic learning experience. Many classes have books, YouTube videos, and other documents not hosted on the site that have been placed together to make a course.
There is also a way for some of the classes to be counted as transferable college credits through certification by the National College Credit Recommendation Service (NCCRS). The requirements for these are specific to each course, but often one has to pay in order to take the final test in order to receive the credit. The acceptance of the credit is also dependent on the individual institutions.They do have a small section for K-12 and they have professional development courses as well. All the courses are reviewed by professors. There is not a requirement to log in to access the resources, but creating a login is required for tracking progress if they want the program to keep track.
Finally, the Library on the site keeps all the resources in the same place in case someone is not looking for a course to take. It has full books that are mostly public domain.