BBC-SOS Teacher
http://www.bbc.co.uk/schools/communities/sosteacher/browse/browse_ks4.shtml
Search the BBC archives for answers to thousands of homework or revision questions, from Key Stage 2 to A level.

BBC-Onion Street
http://www.bbc.co.uk/schools/communities/onionstreet/
An essential study area which offers revision based live chat,daily chatrooms, interactive quizzes and study tips/advice pieces.

BBC Science
http://www.bbc.co.uk/science/
Resources, tips and activities spanning science, space, dinosaurs and robots.

BBCi World Service-Elementary
http:www.bbc.co.uk/worldservice/sci_tech/features/elementary/dyncon/atoms.shtml
Information about elements, atoms and the periodic table.

GcseVise
http://www.gcsevise.com
This site gives a detailed summary of the knowledge requirements for science at key stage 4 of the National Curriculum. Good for revision purposes but you will have to pay to become a member to access further material.