IGOs, Intergovernmental Organizations, are important actors in international affairs. They are composed of sovereign states or other intergovernmental organizations, and their relationships are governed by treaties that give them an international legal personality. They may be (1) worldwide, (2) regional, (3) economic, or (4) structured around cultural, linguistic, ethnic, religious, or historical ties.
Use the Intergovernmental Organization Search Engine (a Google custom search) to search across hundreds of IGO websites.
The following list begins with two general sources that help in finding IGOs, then lists a few very important ones.