CK-12, a great resource for free e-textbook and curricular resources, has just announced that there are now over 100,000 teacher-created digital textbooks available online on their site. These new "Flexbooks" were created by the 30,000 schools that are using CK-12's free digital resources.
They also have some great physics simulations and other resources available, as well as new elementary math resources and PLIX (Play, Learning, Interact, eXplore) which provides an interactive and immersive experience where students learn by doing, allowing them to use critical thinking and creativity.
All of these resources are free and easy to use. The FlexBooks are customizable by the teacher using them to fit their needs and the needs of their students.
Many schools have adopted these resources to replace traditional textbooks, saving millions of dollars and using those saving to purchase technology resources and other needed items.
CK-12's resources are excellent quality, easy to use, and pedagogically well done.