invaluable, the best site for education on the net video tutorials, all free , concise, user friendly, easy to understand, and not patronising. simplifies even the hardest concepts.
For pre-undergraduate students who want a little more stretch than the UK A-Levels offer, I'd recommend this website: http://www.askiitians.com/iit-study-material/iit-jee-chemistry/. It covers a great deal of material in a comparatively advanced manner, and I'd recommend also doing some of the papers of the IIT-JEE and learning how to solve "problems" rather than standard routine plugging in of numbers through the website. The problems asked are of a similar type to those found in the International Chemistry Olympiad, though somewhat less difficult.