The Khan Academy offers a free online collection of more than 3,200 videos on mathematics, science, history and economics. Aside from being free, the best thing about the tutorials is that they are brief -- each is only 10 to 15 minutes long. These to-the-point sessions focus on core concepts and topics covered on national examinations such as the A.P. test and SATs. Most of K-12 math is covered in the sessions, as are key fields of science such as Biology and Chemistry, making them ideal supplements for any student, and worthwhile reviews for anyone. All of the tutorials are taught by Sal Khan, who founded the project. "I believe that someone who experiences the joy and satisfaction of true understanding will never again be satisfied with the superficial type of learning that most students have grown accustomed to," Khan said. "The Khan Academy is about placing deep understanding above anything else."
To get started, click on an area of interest directly from The Khan Academy site, then choose a topic, then choose a lesson (e.g. Math --> Arithmetic --> Basic Addition). Or watch the videos on YouTube.