Connexions: A place for teachers, students, and professionals to search and contribute scholarly content, organized into “modules” or topic areas instead of entire textbooks.
CK12 FlexBooks: A nonprofit that aims to reduce the cost of textbook materials by encouraging the development of what they call the “FlexBook.” Anyone can view or help create these standards-based, customizable, collaborative texts.
Shmoop: An up-and-coming collection of freely shared, expert-written content (most Shmoop authors are Ph.D.s and high school or college-level educators) with the goal of inspiring students and providing tons of free resources to teachers that include writing guides, analyses, and discussions.
MIT Open CourseWare: The Massachusetts Institute of Technology publishes nearly all of its course content on this site, from videos to lecture notes to exams, all free of charge and open to the public. Many other universities are doing the same, often using the content management system EduCommons.