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
Regional Commissions for Economic and Social Development:
Economic Commission for Latin American and the Caribbean (ECLAC)
Economic and Social Commission for Asia and the Pacific (ESCAP)
Economic and Social Commission for Western Asia (ESCWA)
United Nations Economic Commission for Africa (ECA)
United Nations Economic Commission for Europe (ECE)