NOAA, Office of Global Programs, Gloabl Carbon Cycle

U.S. Global Change Research Program, Carbon Cycle Science Program
An Interagency site that contains lots of links, publications,
etc.

NOAA, Climate Monitoring & Diagnostics Laboratory, Carbon Cycle
Greenhouse Gases
Includes sources of data(towers and aircraft data) and figures.

Carbon Dioxide Information Analysis Center, Department of Energy
Data source

USGS Carbon Cycle Research

U.S. Global Change Research Information Office
Includes "Ask Dr. Global Change" :-)

NonProfits
Center for the study of Carbon Dioxide and Global Change
Includes an online weekly magazine, available via email.

University Research Groups
Harvard Forest Home Page

Policy related
The actual Kyoto Protocol