Some useful and fun links:

BBC GCSE Bitesize Useful pages for GCSE revision
MathsNet A great site with games and revision aids
Maths is Good for You Looking at the interesting history of mathematics and relating it to the national strategy
Maths is Fun Lots of really good activities, tests and quizzes.
CGP Books Offering an excellent range of revision guides for KS3, KS4 for all subjects.
Count On A really professional site with too many activities to mention. Not one for dial-up users!
10 ticks A commercial site offering resources to buy.
AQA Assessment Material If you want to practice past GCSE papers on the AQA course, here they are! Please be aware that this is the 3 tier specification, now replaced by the current 2-tier system (requires Adobe Acrobat Reader)
Fractals Some amazing images created by mathematical principles
The Golden Section Read about the principles of the divine proportion (employed in the design of this website)