The Intergovernmental Panel on Climate Change

http://www.ipcc.ch/index.htm

World Climate Research Programme - Climate Variability and Predictability (CLIVAR)

United States Global Research Program (formerly United States Climate Change Science Program)
http://www.globalchange.gov/

World Meteorological Organization

The United States Department of Energy Climate Change Prediction Program is one of the sponsors of the IDAG group:
http://www.sc.doe.gov/ober/CCRD/model.html

The National Oceanic and Atmospheric Administration's Office of Oceanic and Atmospheric Research also sponsors the IDAG group research:
http://www.research.noaa.gov/climate/