Although many organizations were created after World War II (particularly depending on how far past the end of the war we are talking about), I believe that the three most important were the United Nations, the International Monetary Fund, and the World Bank. These three organizations were intended to bring about peace and economic development in the world.
The IMF and the World Bank were created at the Bretton Woods Conference of 1944 (before the war ended, but the organizations really came into existence after the war). The IMF was meant to pool money from various countries to be lent out to countries that needed loans so as to be able to meet short-term commitments. The World Bank was meant to provide countries with loans to help them with projects for economic development. The idea behind these was to create a system that would help all countries develop economically.
The most famous organization created after WWII was the United Nations. This was a successor to the League of Nations. Its main function was to do various things that would help to ensure peace and to prevent another world war from erupting in the future. Thus, an international system was set up to try to ensure peace and prosperity in the post-war world.