Maureen LaFleche spotted this great resource and here's what she has to say about it "Check out CK-12.org. It is a website that provides open-source content and technology tools to help teachers provide learning opportunities for students globally. Free access to high-quality, customizable educational content in multiple modalities suited to multiple student learning styles and levels, will allow teachers, students and others to innovate and experiment with new models of learning. CK-12 helps students and teachers alike by enabling rapid customization and experimentation of teaching and learning styles. Text can be read at two different levels - grade level and basic. You can download books and activities as pdf, epub or mobi." I have checked out several of the units and they look excellent for middle and high school.