Khan Academy offers practice exercises, instructional videos, and a personalized learning dashboard that empower learners to study at their own pace in and outside of the classroom. This service can also be a great ally to parents who try to help their children with homework but have forgotten some of the basics.

Through thousands of interactive resources and lessons relevant to the K-12 curriculum, CK-12 aims to unlock the learning potential within every student, no matter the subject, age or ability. The service is free with registration.

Rather than trying to be all things to all people, this well-edited site focuses on highlighting best-of-breed resources, ideas for interdisciplinary learning, and on being a clearinghouse for educator concerns and needs.