Alley Cat Allies: a national nonprofit clearinghouse for information on feral and stray cats. For over a decade, Alley Cats has advocated trap-neuter-return (TNR) as a humane and effective method to reduce feral cat populations.
American Humane Association: over a century old (founded in 1877), its mission is "to prevent cruelty, abuse, neglect, and exploitation of children and animals and to assure that their interests and well-being are fully, effectively, and humanely guaranteed by an aware and caring society."
American Society for the Prevention of Cruelty to Animals: over a century old (founded in 1866), its mission is "to prevent cruelty and alleviate the pain, fear, and suffering of animals, through nationwide education, awareness and legislative programs."
Animal Concerns Community: since 1991, this project of the non-profit EnviroLink Network serves as a clearinghouse and provides internet access to thousands of environmental, animal rights, and animal welfare resources.
Animal Welfare Institute: "Since 1951, the Animal Welfare Institute has been dedicated to reducing animal suffering caused by people. We seek better treatment of animals everywhere - in the laboratory, on the farm, in commerce, at home, and in the wild."
Defenders of Animals: founded in 1978, this organization's mission is "to provide assistance to sick, injured and homeless animals and to defend the inalienable rights of both companion animals and wildlife through education, legal and legislative activism.
Humane Society of the United States: founded in 1954, HSUS envisions a world in which people satisfy the physical and emotional needs of domestic animals; protect wild animals and their environments; and change their relationship with other animals, evolving from exploitation and harm to respect and compassion.
Wildlife In Crisis (WIC): "Wildlife in Crisis (WIC) is a non-profit organization dedicated to wildlife preservation and land conservation. WIC rescues injured and orphaned wildlife and seeks to protect threatened ecosystems."
National Council on Pet Population Study and Policy: among the goals is "to serve as a national collection point for gathering and evaluating available pet population data and relevant materials."
World Animal Net: largest network of animal protection societies with over 1,700 affiliates in more than 90 countries.