(D), (E), (F), (Spa), (Ru) / http://www.imo-official.com/
The official IMO-website, containing results, statistics and problems of all IMO competitions since the very first in Romania in 1959.
(E), (F) / http://imo.math.ca/
A Canadian page containing a lot of information about the International Mathematical Olympiad. Besides links to the websites of past competitions there is also a comprehensive collection of problems.
An unofficial page that collects all results of the IMOs since 1982 and displays them tabularly.
A site containing my problems from diverse mathematical competitions (amongst others all problems from the Austrian-Polish competition), often with solutions. (The site went offline, but luckily it is still available in the Web Archive.)
(E) / http://www.mathlinks.ro/
A very well done page about mathematics and mathematical competitions. It contains a huge bulletin board, many problems and lots of interesting and useful links. Additionally there is a good introduction to LaTeX (which is also used in our Wiki).
(D) / http://raach.info/
The homepage of the "Seminarzentrum Raach am Hochgebirge", where the annual preparation courses for the ÖMO take place. (only in German)
(E) / http://www.imomath.com/
A comprehensive site about the mathematical Olympiad. It offers news, a bulletin board and many links. Additionally a part of the site is dedicated to a book about all problems suggested for the IMOs (the so-called short- and long-lists).
Information about past IMOs. With problems, logos and regulations. In addition there are many links to national, regional and international mathematical competitions.
(D), (E) / http://www.mathespass.at/
A page full of mathematical problems (including a "problem of the month") run my an Austrian olympian. Most problems are not too difficult and aim at sparking interest in the mathematical olympiade.