Khan Academy offers a free online collection of more than 5,000 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, conversational sessions focus on core concepts and topics covered on national examinations such as the A.P. test and SATs. All of the tutorials are taught by Salman 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."

You can watch the videos on YouTube, and follow Khan Academy on Facebook and Twitter.