CK-12
The biggest open content provider for K-12 STEM content, CK-12 provides standards-aligned teaching resources. Content is mapped to a variety of levels and standards, including common core. The most incredible feature of CK-12 is its customizable textbook. You can customize a textbook (called "Flexbook") for your students to use. You can start from scratch or build from anything in the FlexBooks library.
