http://www.mathsnet.net/  
Features include: Interactive, Articles, QuizBank, Magic Grid, MathsNet Art, Puzzles and Maths Links

http://www.mymaths.co.uk/  
This popular website includes online lessons, games, tests and over 300 teacher made worksheets. Students can also sign up for Puzzle of the Week

http://www.mathsisfun.com/

A great website to explain the basics in mathematics

http://www.bbc.co.uk/radio4/science/5numbers.shtml

A great selection of audio files by Simon Singh for Radio 4.  The great master of mathematics talks about various fascinating topics and engages the audience in an exciting and accessible way.

http://www.websudoku.com/

So you like sudoku?  Try it online!  Available in easy, medium, hard and evil levels.

http://www.edexcel.org.uk/quals/gcse/maths

The latest information about the new two-tier GCSE in mathematics

http://tinyurl.com/mf9lm

An excellent selection of books by the great puzzle master Raymond Smullyan