Link to the official Asian Pacific Mathematics Olympiad (APMO) website.
A great community of problem solver and a wealth of resource for the students, and problem solvers of Bangladesh. It is the very first math forum of Bangladesh with equation writing (LaTeX) support. If you are participating BdMO or serious about problem solving this website is a must-join for you!
This website is aimed at inspiring young people who are interested in math. Its geometry section is very rich.
One can find the theorems and problems of geometry with interactive java applets.
An awesome journal for young problem solvers
It is a very large archive of the problems of different math Olympiads around the world. Most importantly, you can find the solutions of almost all the problems of this achieve and discuss them in the forum.
Another very good encyclopedia of mathematics.
Wikipedia is not only the biggest encyclopedia but also is the largest reference for the mathematical theorems, formulas and their proofs.
An excellent journal for problem solvers. It is a must read online journal for every problem solver. It is edited by the legendary math Olympiad trainer, Dr. Titu Andreescu
Maths for kids is a mathematics website made for children from 6 to 11 : they can choose between different levels and exercises : additions, subtractions, multiplications, divisions, numbers, measuring exercises and mathematical problems.
Every hour, there is a new competition, and the winner can select his country, or a country he likes. In this way, children can improve their mathematical level in the run up to the Olympics Games.