Khan Academy is a non-profit educational resource composed mainly of instructional videos and practice activities. These videos and activities span many subject areas and topics.
If you create an account as a student, the site can also track your progress and guide you through learning different skills. Teachers can also create accounts to track and assign topics to students.
A new extremely useful feature that has been added is the Common Core map - this covers grades K-8 as well as High School topics in Math. Even if the instructional videos (hosted on YouYube) are blocked, the practice sets are a wonderful resource to practice Math skills.
...And if you are interested in the origins and goals of Khan Academy, the TED talk by Salman Khan is a great place to start.