S-cool is by far one of the best free sites out there. It has loads of subjects and more important, the notes are concise.
As an A level student, I believe that the notes provided from the A level section are excellent.
The notes have flash videos in them, that make them even more effective. Although it mainly focuses on the UK boards way of classifying chapter, I can assure you that the topics can easily be related to an international syllabus, like CIEs.
The Link to the website
