Newsgroups Catster and Dogster link up animal lovers across the continent to help move homeless pets from overpopulated areas to parts where spay-neuter programs have reduced the number of homeless cats and dogs. Click on the link to this post to read a nice article from the Boston Globe about a new underground railroad (well, not underground at all) moving unwanted animals from the midwest and south which has too many animals in shelters, to areas with space in shelters and loving homes waiting to adopt a pet. This railroad is moving cats and dogs from shelters where they would otherwise be euthanized, so it's quite an important volunteer group.
Catster 9 Lives Adoption page (source of the kitty picture). See also Dogster site.