The Open Textbook movement focuses on the creation of books that are built specifically for use as free or low-cost options for education. Many of the collections will have links to the same books, but each repository has a particular focus, and items you can't find in other collections. While there are other lower cost options out there, this list focuses exclusively on freely available content.
MIT OpenCourseWare (OCW) is a web-based publication of virtually all MIT course content. OCW is open and available to the world and is a permanent MIT activity. Materials are available from over 2400 courses all MIT departments.