Sites: 
National/International Standing
Official  & Research Sites with General Educational Sections. 
Other Climate Centers and  Authoritative Sites
UNEP
Largest International Climate Resource
The WMO climate section is written in clear language. The WMO annual Greenhouse Gas Bulletin is an important report on the state of the climate.
International Geosphere Biosphere Project
Closest thing to looking at the whole Earth. Has a Climate Change Index.
World Health Organization
Fact sheets, Slide show. 
The US National Snow and Ice Data Center
The Arctic's responses to climate change are the sources of planetary catastrophe and determine the fate of the northern hemisphere and entire planet. 
METED  National Environmental Foundation: Climate Change Fitting the Pieces Together
Best site overall for the basic science. Well presented as video and module.
Global Carbon Project
The authoritative site project for global carbon emissions and carbon sinks

National Research Council Climate Stabilization Targets 
The latest (2010)  and best global climate change assessment. For those with basic science knowledge.
The Intergovernmental Panel on Climate Change
Not the best site for basic learning. The full Synthesis Report is more for those with climate knowledge. It is by far the best IPCC report.
 Best is the FAQ section.

The Royal Society: Climate Change - A summary of the science
Climate change science statement: authoritative statement on what is 100% certain, as a response to  skeptics campaign

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European Commissions Climate Action
A good site for general science as well as Europe. The EU leads on policy and proposals.
The 2006 Stern Commission Review on the Economics of Climate Change
Still a top climate change assessment as well as the economics. 
UN Climate Convention (FCCC) Secretariat.
Brief Introduction to climate change is a top source. 
Tyndall Center UK Climate Change
Research projects on impacts
Koshland Science Museum of the National Science Academy
Excellent educational site from its climate change exhibit.
British Antarctic Survey
Ice cores provide best evidence of climate sensitivity to warming
World Climate Research Program
Scientist's site for coordinating research projects.

National Academies Press
Large section of climate change online books and report - the older publications are still good

Encyclopedia of Earth
Unique & excellent site developed by experts for general readership. 
UK Government Office for Science
Excellent presentation of the science. Concise and in some depth. 
Australian Government Climate Commission.
The Critical Decade by Australia's top scientists, 2011. Global & International
Harvard Medical School Center for Health and Global Environment
Top site on human health aspects. Past lectures, papers, research.
World Resources Institute
NGO government recognized resource
Scripps Institute of Oceanography
(US) Superb Earth education site with teachers resources.
Smithsonian Forces of Change
Atmosphere design lab -a top interactive. The climate change section is not longer on line. 
UN Climate Change Kit
(2001) still relevant.  Condensed version for public - broad coverage including UN climate convention and Kyoto