Humane Organizations, Shelters and Sanctuaries and other Nonprofits

American Temperament Testing Society Tests purebred and mixed breed dogs to a uniform temperament standard
Animal Place
ASPCA The American Society for the Prevention of Cruelty to Animals
Best Friends Animal Sanctuary
Black Beauty Ranch
Farm Sanctuary
Hugs for Homeless Animals
     Worldwide shelter list
HSUS  The Human Society of the US
RSPCA The Royal Society for the Prevention of Cruelty to Animals
Tree House Animal Foundation
1-800-save-a-pet.com A nonprofit organization advertising and encouraging adoption.

Wildlife and Conservation Organizations

African Wildlife Foundation
Arkive
Born Free Foundation
Conservation International
Cousteau Society
Defenders of Wildlife
Dian Fossey Gorilla Fund
Ducks Unlimited
Fauna and Floral International
Gorilla Foundation, The
Greenpeace
Jane Goodall Institute, The
National Audubon Society
National Parks Conservation Association
National Wildlife Federation
Nature Conservancy, The
Project Feederwatch
Sea Shepherd Conservation Society
World Wildlife Fund

Service organizations

Assistance Dogs International, Inc.
Canine Assistants
Canine Companions for Independence
Canine Partners for Life
Guide Dogs for the Blind
Helping Hands Monkeys for the Disabled
International Association of Assistance
     Dog Partners

Activist and Political Organizations

AAVS American Anti-Vivisection Society
American Humane
Animal Legal Defense Fund
Animal News Center
Animal Protection Institute
Doris Day Animal League
Eurogroup for Animal Welfare
Friends of Animals
Fund for Animals, The
In Defense of Animals
IFAW International Fund for Animal
          Welfare
NAVS National Anti-Vivisection Society
PETA People for the Ethical Treatment of
          Animals
United Animal Nations
    EARS  Emergency Animal Rescue
                 Service
World Animal Net
WSPA World Society for the Protection of
            Animals

International Government Agencies and Organizations

IUCN International Union for Conservation of
         Nature and Natural Resources

US Government Agencies and Organizations

USDA  US Department of Agriculture
   APHIS  Animal and Plant Health
                Inspection Service
        Pet Travel
       Veterinary Services
   Natural Resources Conservation
      Service
   Forest Service

US Department of Health and Human Services
   CDC Centers for Disease Control
       Importation of Pets and Animals
   FDA Food and Drug Administration
       Center for Veterinary Medicine
       Greenbook of Approved Drugs
   National Institutes of Health
       Office of Animal Care and Use
 
US Department of the Interior
   National Park Service
   US Fish & Wildlife Service
   US Bureau of Land Management