National:

African American Environmentalist Association   http://www.aaenvironment.com/

Alliance for Climate Protection   http://www.wecansolveit.org/content/homepage/

Alliance to Save Energy   http://www.ase.org/

American Bird Conservancy    http://www.abcbirds.org/index.html

American Farmland Trust     http://www.farmland.org/

Climate Solutions    http://www.climatesolutions.org/

Defenders of Wildlife   http://www.defenders.org/index.php

Earth First!  http://www.earthfirst.org/

Earth Island Institute   http://www.earthisland.org/index.php

Environmental Defense Fund http://www.edf.org/home.cfm

Environmental Law Institute   http://www.eli.org/

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

Green Building Initiative   http://www.thegbi.org/home.asp

Green Building Institute   http://www.greenbuildinginstitute.org/

Greenpeace  http://www.greenpeace.org/usa/

Izaak Walton League  http://www.iwla.org/

League of Conservation Voters   http://www.lcv.org/

Rainforest Action Network     http://www.ran.org/

Sierra Club   http://www.sierraclub.org/

Student Environmental Action Coalition (SEAC) http://www.seac.org/

The Nature Conservancy  http://www.nature.org/

The Marine Mammal Center    http://www.marinemammalcenter.org/

United States Green Building Council  http://www.usgbc.org/

World Wildlife Fund  http://www.worldwildlife.org/

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

National Wildlife Federation   http://www.nwf.org/

The National Geographic Society  http://www.nationalgeographic.com/

National Recycling Coalition  http://www.nrc-recycle.org/  

National Audubon Society   http://www.audubon.org/

Rising Tide North America http://www.risingtidenorthamerica.org/wordpress/category/front-page/

The Wilderness Society  http://www.wilderness.org/

WorldChanging http://www.worldchanging.com/

Union of Concerned Scientists   http://www.ucsusa.org/

Association of Environmental Professionals

Center for a New American Dream

Center for Progressive Reform

Clamshell Alliance

Clear the Air (United States)

Earth Liberation Army (ELA)

Earth Liberation Front (ELF)

Ecologyfund.com

Environmental Life Force (ELF)

Forest Guardians

Global Green USA

High Uintas Preservation Council

Institute of Environmental Sciences and Technology

Native Forest Council

Nature's Classroom

Negative Population Growth

Neighborhood Parks Council

Population Connection

Population-Environment Balance

Republicans for Environmental Protection

Student Conservation Association

Texas Campaign for the Environment

The School for Field Studies  

Waterkeeper Alliance

Worldwatch Institute

International: 

Center for International Environmental Law  http://www.ciel.org/

Earth Easy  http://www.eartheasy.com/

Earth First  http://www.earthfirst.org/

Earth Island Institute   http://www.earthisland.org/index.php

Eco World  http://www.ecoworld.com/default.cfm

Environmental News Network  http://www.enn.com/

Env Voters  http://environmentvoters.org/

Friends of the Earth International   http://www.foei.org/

Global Environment Facility   http://gefweb.org/index.html

Green Cross International  http://www.greencrossinternational.net/index1.html

Greenpeace International  http://www.greenpeace.org/international/

IUCN The World Conservation Union

International Institute for Sustainable Development  http://www.iisd.org/default.asp

The Nature Conservancy  http://www.nature.org/

The Sierra Legal Defense Fund  http://www.sierralegal.org/

United Nations Environment Network   http://www.unep.net/

United Nations Human Settlement Programme  http://www.unchs.org/

World Wildlife Fund    http://www.panda.org/

World Conservation Union  http://www.iucn.org/

World Business Council for Sustainable Development   http://www.wbcsd.ch/

Biofuelwatch

Biosphere Expeditions

BirdLife International

Center for International Environmental Law

Conservation International

Conservation Law Foundation

Earth Charter Initiative

Earth Policy Institute

Earthwatch

Environmental Law Foundation (ELF)

Environmental Investigation Agency http://www.eia-international.org/index.shtml

Forest Stewardship Council

Friends of Nature

Friends of the Earth

Gaia Mater (the mother Earth)

Global Water Policy Project

Global Witness

Global Vision International

Great Transition Initiative

International Analog Forestry Network

International Institute for Sustainable Development

International Network for Sustainable Energy (INFORSE)

Regenesis - the Global Movement for Environment Sustainability, Liberty & Social Justice

Stockholm Environment Institute (SEI)

Wetlands International

Whale and Dolphin Conservation Society

Wildlife Conservation Society

World Business Council for Sustainable Development

Worldchanging

World Conservation Union (IUCN)

World Resources Institute (WRI)

Worldwatch Institute

Yellowstone to Yukon Conservation Initiative

There are also a large number of regional organizations and national organizations for other countries across the globe that have not been listed here.