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
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.