US and Canada
Adopt-a-Pet.com (formerly 1-800-Save-A-Pet.com): A non-profit pet adoption charity that helps shelters, humane societies, SPCAs, pet rescue groups, and pet adoption agencies advertise their homeless pets to adopters for free.
Directory of No-Kill Shelters: This site lists numerous no-kill shelters throughout the US (and a few in Canada). If you know of others that are not listed, why not e-mail them and let them know so they can add that shelter to their directory?
Petfinder: An on-line, searchable database of animals that need homes. It is also a directory of over 10,000 animal shelters and adoption organizations across the USA, Canada and Mexico. Organizations maintain their own home pages and available pet database.
United Kingdom
RSPCA: The RSPCA works to prevent cruelty, promote kindness to and alleviate suffering of animals. It also operates shelters throughout the UK.
Australia
RSPCA: Each state has its own RSPCA branch, but this is the main site for RSPCA Australia.
Worldwide
Purebred Cat Rescue: The Purebed Cat Breed Rescue website has a mission to support responsible purebred cat rescue efforts. Purebred cat rescue involves the rescue of both pedigreed cats and “copycats.” Purebred rescue organizations are usually composed of people who are expert in identifying the various breeds of cat and dealing with the specific needs of the breed(s) that they rescue. They aim to not only promote and rescue purebred cats, but also endeavor to place these purebred cats into caring, loving homes for adoption.