Gooru is a new (alpha) site I learned about from Richard Byrne on his Free Technology for Teacher's Blog.
Gooru is still in alpha status, meaning it is not fully open to the public yet, but you can still sign up. Gooru is an online study tool that allows students to explore resources and study guides that are aligned to standards. The materials are in math and science and are from 5th grade up to high school. Resources include digital textbooks, videos, animations, and much more. It also includes social media functions so that students can work, study, and share with others.
There are self-assessments in Gooru that help it adapt to the student, suggesting resources and study guides based on the student's performance.
The resources are all vetted by educational professionals and you can even create custom collections and resources.
The site has resources to learn more about it and how it can be beneficial to educators and students.