There are a number of free academic support sites that will help students on a wide range of subjects.

Khan Academy : With over 3,200 videos on everything from arithmetic to physics, finance, and history and hundreds of skills to practice, we're on a mission to help you learn what you want, when you want, at your own pace.

Hippocampus: This site has full courses with activities, videos, lessons, and questions for review for a wide range of courses including AP courses.