Khan Academy is completely free and available to anyone that wants to use it. To use it click learner on the home page, then sign up for a free account, and lastly search for your subject!
Homework Spot has every subject known to man available to help you learn and study. All you have to do is click the link below and click the subject you want!
iCivics can help you study for your next history exam, or even just teach you about the U.S. Government and help you learn more about how the world works and help you be a better citizen.
Science Tech has an interactive website that allows you to learn about all things Science related. They keep up to date with the latest scientific updates.
Kids Gov is the official kid's portal for the U.S. Government. It links to over 2,000 government agencies, schools, and educational organizations, all geared to the learning level and interest of kids.