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.
And this just names a few. Some schools and districts are also relying more and more heavily on open source content: The Bering Strait School District (BSSD) in northwest Alaska, for instance, created an entire K-12 curriculum through their "Open Content Initiative," which now has 14,000 pages and growing. Teachers, administrators, visitors, and even students can contribute, and all is Creative Commons licensed.
Is this a great idea, or kind of frightening?
Some argue that too heavy a reliance on open source software is dubious, since curriculum is based on a consistent approach unique to each teacher. But the beauty of these resources is that not only are they accessible, they're customizable, and the bounty alone is pretty exciting.
Educators are taking note that a collective of practitioners may know more than a minority of experts (case in point: even Ivy League universities are embracing the use of Wikipedia in education). Maybe the honor system -- also known as Web 2.0 -- has become far too useful to fail.