Veterinary
International Veterinary Information Service
This site contains several full-text veterinary books including Recent Advances in Companion Animal Behavior Problems (an issue by Professor K. A. Houpt, DVM Dipl ACVB. You have to register (and you need to use a real e-mail address), but registration is free.
UC Davis School of Veterinary Medicine
This is a link to the school's search engine. They have some bibliographies to help get you started when researching a particular behavior. For example, I was researching barking and found a twelve-page list of references, as well as a literature review.
Veterinary Education Information Network
The University of Sydney's Veterinary School's website.
VeterinaryPartner.com
A site run by veterinarians. This contains articles tha covers animal behavioral and medical issues.
ExoticPetVet.com
The website of an avian and exotic animal hospital in Chicago. This site contains care sheets for various exotic pets.