I maintain an online catalog of books whose authors have intentionally made them free (as opposed to what Project Gutenberg does with old public domain books). Probably a majority of these are textbooks.
In my experience, nearly all free textbooks are simply written by their authors without working through any organization. Often professors write up lecture notes, and the lecture notes gradually become more and more elaborate, and evolve into a book. During the ~14 years that I've been paying attention, I've seen many initiatives, consortia, and for-profit companies come and go. Most have had very little impact. A good example of a zero-impact initiative was California Governor Schwarzenegger's Free Digital Textbook Initiative, which was announced with great fanfare and accomplished absolutely nothing. (There is a newer initiative called the California Open Education Resources Council.)
A couple of organizations that have done good work are merlot.org and OpenStax.