Here are some of the most important and valuable links available:
Climate Change - Web Resources: Links to official sites, research organisations and campaign groups

UN Framework Convention on Climate Change http://www.unfccc.de/

United Nations Environment Programme http://www.unep.org

UNEP documents on climate change
http://www.unep.org/search/unep.asp?q=climate+change

Intergovernmental Panel on Climate Change IPCC Third Assessment Report
Working Group I - Draft Outline
http://www.met-office.gov.uk/sec5/CR_div/ipcc/wg1/wg1-outl.html

U.S. Environmental Protection Agency: Global Warming
http://www.epa.gov/globalwarming/

Meteorological Office http://www.meto.govt.uk/

Extreme weather http://www.met-office.gov.uk/ukclimate/extremes.html

Friends of the Earth http://www.foe.org/

Friends of the Earth Save the Climate Treaty
http://www.foe.org/international/climate.html

Greenpeace http://www.greenpeace.org/

Greenpeace's International Campaign to Save the Climate
http://www.greenpeace.org/~climate/

Worldwatch Institute
http://www.worldwatch.org

WWF-UK Climate Change Campaign
http://www.wwf-uk.org/researcher/issues/climatechange/index.asp

Center for the Study of Carbon Dioxide and Global Change
http://www.co2science.org/

Natural Resources Defence Council http://www.nrdc.org/

World Climate Report (skeptic's view)
http://www.greeningearthsociety.org/climate/

Global Change - published by Pacific Institute for Studies in Development, Environment, and Security http://www.globalchange.org/default.htm

Climate Ark http://www.climateark.org

Rising Tide Festival of Resistance http://www.aseed.net/climate/climate.htm
and
http://squat.net/climate/action.htm

Equity Watch Newsletter - from the Delhi-based Centre for Science and the Environment http://www.cseindia.org/html/cmp/climate/ew/index.html

NCDC (Nat. Climatic Data Center). Global warming FAQ. Nat. Oceanic
and Atmospheric Admin. (NOAA). 19 Dec 2001: pp 6. Web site:
http://lwf.ncdc.noaa.gov/oa/climate/globalwarming.html

NOAA (Nat. Oceanic and Atmospheric Administration). Climate impact
of quadrupling atmospheric CO2 - an overview of GFDL climate model
results. GFDL web site:
http://www.gfdl.gov/~tk/climate_dynamics/climate_impact_webpage.html

Tree Rings: A study of climate change at:
http://vathena.arc.nasa.gov/curric/land/global/treestel.html

http://yosemite.epa.gov/oar/globalwarming.nsf/content/ResourceCenter.html

New Scientist magazine provides a huge set of excellent resources at the following url:
http://www.newscientist.com/hottopics/climate/climatelinks.jsp
http://www.newscientist.com/hottopics/climate/climatescenario.jsp