Website of the Union of International Associations which publishes the Yearbook of International Organizations. It is self-described as a non-profit, apolitical, independent, and nongovernmental organization which has been 'a pioneer in the research, monitoring and provision of information on international organizations, international associations and their global challenges since 1907'. It also contains a link directory to major organizations.
Official website of the UN providing the initial point of navigation for the entire range of the organization’s functions.
Another useful directory of IOs which provides direct links to UN-linked websites as well as those of many regional organizations and NGOs.