All-dog forum focusing on health, nutrition, behavior, grooming, advocacy, etc. Also has horse, cat and ferret sections.
http://forum.dog.com/
Members dicuss animal information, cats, dogs, rodents, certain exotic pets, and more.
http://www.paw-talk.net/index.php?
Has a category for every pet imaginable, even sugar gliders and guinea pigs.
http://www.petshub.com/
Dog chat, news, articles, health care, breeds, grooming, and puppy information.
http://www.chazhound.com/forums
Large pet forum; includes general pet chat, followed by dog, cat, horse, rabbit and small pet categories.
http://www.indulgeyourpet.com/boards/index.php
Sections of pet loss/grief, handicapped pets, pet travel/transportation, pet photos/contests, and more.
http://www.marylandpet.com/pet-forums.htm
Very large forum with pet chat, bird, dog, cat, fish, reptile, small pets and other categories.
http://forums.petlovers.com/vb/
In addition to the regular cat, dog, bird, and other pet categories, this site offers pet loss and pet adoption sections.
http://www.petpeoplesplace.com/
Discussions on birds, cats, chinchillas, dogs, ferrets, hamsters, horses and more.
http://www.petwebsite.com/forums/default.asp
Threads of pet emergencies and health, cat and dog forums, training, breed discussion, and the rainbow bridge -- where pets go after death.
http://www.pets.ca/forum/index.php