ASPCA

Petfinder.com was the first site of its kind and is today the largest single force introducing homeless pets with new families. Launched in 1995, Petfinder's goal is to increase public awareness of the availability of high-quality adoptable pets and the overall effectiveness of pet adoption programs across North America to the extent that the euthanasia of adoptable pets is eliminated.
www.petfinder.com

Find an American Society for the Prevention of Cruelty of Animals (ASPCA) shelter in your area.
http://www.aspca.org/adoption/shelters/