Doris Day Animal Foundation
A nonprofit organization founded in 1998 by legendary performer Doris Day. Dedicated to promoting increased protection for animals, DDAF accomplishes its mission primarily through educational programs that empower others to act on behalf of animals. http://www.ddaf.org/

SPAY/USA
A program of The Pet Savers Foundation, a nationwide network and referral service for affordable spay/neuter services.
http://www.spayusa.org/

American Partnership for Pets
Strongly encourages spay or neuter as a part of responsible pet ownership -- for health and behavioral reasons, and as a means to prevent accidental breedings resulting in unwanted litters.
http://www.americanpartnershipforpets.org/

PaLC: Prevent a Litter Coalition
Dedicated to ending the tragedy of unwanted and homeless pets, resulting in the euthanasia of adoptable animals, by working towards long-term community solutions to align the pet population with available good homes, with guaranteed humane care for their lifetimes, and by providing a forum for effective collaboration between those who share our mission.
http://www.palc.org/

Alley Cat Allies
As the foremost experts on feral cats, promotes nonlethal control for feral and stray cats with trap-neuter-return (TNR) programs that effectively reduce their population by sterilization - not euthanasia.
http://www.alleycat.org/

Pets 911
Nationwide information clearinghouse on pet adoptions, animal shelters, and lost and found.
http://www.pets911.com/