Here is a collection of links that I and my students have found useful. There's a brief description of each link and why I recommend it.
This is the oldest website on here and it's a pretty solid all-rounder. It's well written, has lots of high quality content and diagrams but it suffers from not having been updated thoroughly for the last decade or so and having a poor menu system/site layout.
This website is highly detailed and contains just about everything you need to know. It also contains lots of information that isn't on the course but you could be questioned on as extension work to gain grades 7-9. It does have a couple of drawbacks - the website layour makes it difficult to read.
The AQA website contains the syllabus, practice past papers and lots more. The link above is to the science area. The two links below are specifically for Chemistry past papers:
GCSE Chemistry (Triple Science)
This is my favourite general interest science website of all. It's written in an easily accessible format, has lots of interesting articles on current research and allows you to extend your learning beyond the GCSE course. I would definitely recommend reading this for fun if you intend on taking any A-level sciences.
This is a FREE version of New Scientist. It's written in a more accesible format and is a fair bit simpler so for some students it's more suitable. There do tend to be error/poor journalism here and there but did I mention that it's FREEEEEEE!!!
This is NOT A GCSE website. It's aimed at A-level students, therefore some of it is suitable for students aiming at grades 7-9 who wish to challenge themselves/prepare for A-level.
This one is here because many of my students talk about it! I've had a skim and it looks pretty good - there's a forum that's rather active and several interesting discussions going on.
If you have any suggestions e-mail them to me firstname.lastname@example.org and I'll review them.