For a complete list of all U.N. organizations, see the U.N.'s alphabetic index.
The following are just the ones most relevant to international economics.

International Development Association
International Centre for Settlement of Investment Disputes
International Bank for Reconstruction and Development
International Finance Corporation
International Labor Organization
International Monetary Fund
Multilateral Investment Guarantee Agency
United Nations Conference on Trade and Development
World Bank
World Intellectual Property Organization
World Trade Organization