http://shmoop.com
This site contains test prep materials as of now. They plan to add in online MOOC style classes for 187 different Common Core classes. As of now, they have some quality material for biology but none for chemistry or physics. Some of the material, including the test prep stuff, is behind a paywall. The material is interesting as it is geared totally for test prep, allowing a student to pay for the stuff and then get a higher grade. They even feature an Essay Lab designed to help a student start essays on literature. This feature is behind the paywall but there is a tour.
Overall, the site could help for AP test prep. It will be itneresting to see what they put up for Common Core.
