Khan Academy is a non-profit organization dedicated to providing educational courses and classes to assist students of any year. It was started in 2006 by Salman Khan and continues to be a very useful source of education.
The classes themselves are in the form of Youtube videos explaining the topic and giving examples. The different subjects provided include Math, Science, Economics and finance, Arts and humanities, Computing, and Test prep. Each subject has multiple videos explaining many of the details in each.
Khan Academy also has a points system that rises with each video that you complete. Practice quizzes are also an option.
If you are in need of some extra learning, you can sign up for Khan Academy free at any time.