The Art of Problem Solving (AoPS) website,1 established in 2003, has
grown to over 29,000 members.2 We believe that it is the largest website
of its kind in the English-speaking world, with mathematics resources
developed specifically for high-ability middle and high school students.
AoPS has been called “a revolution in mathematics training for the top
high school students.” 
2 Forum, Blogs, and Wiki
The heart of AoPS is the AoPS Forum, which has over 800,000 posts
on a variety of topics, mathematical and otherwise. The AoPS Forum
is for students, parents, and teachers to discuss various math problems and other topics of interest to people interested in math. The Forum
is free for everyone, and its members are students and teachers from all
different ages, locations, and abilities.