http://www.ck12.org/browse/
This site contains online learning materials. I browsed chemistry. The chemistry heading has 8 headings and each heading has a number of concepts. Each concept has 1 or more modalities. Clicking on a modality brings up something to read, often with pictures, and sometimes with youtube videos and even some questions at the end.
Biology has 9 headings and many more concepts than chem. They also have physics, life science, and physical science. The material I looked at is clear and appealing to the eye.
This site would be an excellent reteaching resource for students identified as in need of help by formative assessment. It could also serve as an online textbook type resource.
