This is formative assessment website which provides guidance for assessment in maths along with 100 formative assessment lessons. A great website all round!
This Canadian site has information on the maths olympiad and links to other sites.
Plus+ is a complementary online magazine, which to tell students about the diverse applications of mathematics to solve problems in physics, chemistry, biology, engineering, and economics. It provides stories about new developments in mathematics and mathematical sciences, interviews with mathematicians, information about degree courses, history of mathematics and science, mathematical biographies, and links to other mathematics websites and resources.
This website has lots of maths links, particularly to geometry.
Maths games, maths lessons, maths puzzles and maths fun for kids of all ages.
This website is developed and maintained by Rod Pierce, who loves mathematics and fun. The idea behind the site is to offer mathematics as well as some fun bits, and to combine the two wherever possible.
This website provides rich problem solving activities for both primary and secondary students and teachers
This american website has lots of puzzles that focus on problem solving. They are great to use in class!