Animaland.org -- Snazzy site run by the ASCPCA (American Society for the Prevention of Cruelty to Animals), includes a helpful animal encyclopedia and pet care guides organized by animal (from fish to rats).
Healthy Pet.com -- The American Animal Hospital Association (AAHA) has dog, cat, and exotic categories, featuring articles on standard pet care topics (nutrition, illness, behavior), plus some interesting insights on animal/human bonding. Includes a coloring page for kids and a hospital finder.
AVMA's Care for Pets -- The American Veterinary Medical Association covers the pet companionship experience from selecting a pet to coping with loss.
General Pet Info
Petsyclopedia -- Complete sections on dogs, cats, guinea pigs, rabbits, hamsters, and pygmy goats.
CentralPets.com -- Has some information (or links to other resources) about most animals in captivity. Marsupials. Newts. Skunks. All here, right alongside the dogs and parrots.
Specialty -- Dogs and Cats
PetPlace.com -- Extensive, highly detailed libraries of information on dogs and cats.
Lovethatcat.com (retail site) -- A listing of low cost and free spay-neutering programs around the country.
Online Community
TerrificPets.com -- For dog, cat and horse lovers: Chat rooms, memorial forum for pets that have passed away, and some articles.
PawsperousPets.com: "The Pet Lovers Meeting Place:" Personal pet stories, pet humor, pet tips, and info for renters with pets.
Supplies
Epinions Reviews -- More than 2,000 reviews of supplies for dogs, cats, bird, and fish.
Health
Fighting Fleas and Ticks -- From the Center for Disease Control: How to keep your pets healthy and prevent the spread of infectious diseases.
A Guide to Pet Care for the Immunocompromised -- From Ohio State University, a very short guide for pet selection and pet care for HIV/AIDS and chemotherapy patients, the elderly, and small children.
The Pet Center -- Go to "The Internet Animal Hospital" for everything about surgery, disease, and health: x-rays, surgery photos, quizzes, pharmaceutical info, etc.
Travel
Pets Welcome -- Find lodging, exchange info, read tips on travelling with your pet.
Pets on the Go -- Listings and reviews of pet friendly lodgings.
Adoption
Petfinder.com -- An extensive database of pets that need homes and a database of more than 9,000 animal shelters and adoption organizations.
PETS 911 –- Animal Hospital's pet adoption and assistance site.
Pets with Disabilities -- Promotes the adoption of disabled pets (mostly dogs and cats), and has stories, foster programs, and a directory.
Memorial
In Memory Of Pets -- Stories, poems, prayers, and photos of deceased pets.
Virtual Pets
NeoPets -- If you don't want the fuss of a real thing, create a virtual fantasy pet and join a network of people who probably still own Tamagotchis. But, seriously, the games seem fun.
Cute Pet Photos
Cute Overload -- Yep, just lots of very cute pictures of adorable dogs.