Below are links to some sites that I personally have visited and found helpful. If anyone knows of any other good sites, please e-mail me and if I think they are useful I will put them on this page.
http://www.8886.co.uk - this is a brilliant website on GCSE coursework. There is an example investigation which is written up for you to follow and a mark guide which shows you how to hit all the relevant points. This site is written mainly for Higher students aiming forA*-B grades, but there is also some useful material for Foundation students.
http://www.revise.it - this site is more suited to AS level than GCSE, but there is some useful stuff for higher ability students. The science topics are split up into Biology, Chemistry and Physics.
http://www.schofieldandsims.co.uk - this is a site which sells posters online (including science ones). The periodic table poster is one of the best I've seen. Highly recommended.
You can buy GCSE science books and revision guides online at amazon.com - just click on the link below.