CK-12From the website: "Using an open-source, collaborative, and web-based compilation model, CK-12 pioneers and promotes the creation and distribution of high-quality, adaptive online textbooks that can be mixed, modified and printed (i.e., the FlexBook™ textbooks). The Site is designed to serve as both an information resource and an application platform for K-12 teachers, educators and students who wish to join our online community."