The best on-line source for pet owners is Veterinary Partner, by the Veterinary Information Network.  Below are sites we have found particularly interesting or useful.

Animal Poison Control Center

Animal Protection, by the Animal Protection Institute in Sacramento.

Asthma in Cats; also, check out aerokat.

Diabetes in Cats

Canine Epilepsy, hosted by the University of Missouri.

Feline Health, hosted by Cornell University.

First Aid, hosted by Colorado State University.

Gerbils, National Gerbil Society of the United Kingdom.

Ophthalmology

PKD

Poisonous Plants

Reptile Husbandry

Senior Dog Project

Training and Clicker Training

Travel

Wild Bird Care