AAVMC Veterinary Schools in the US
A direct link This is a link to the Association of American Veterinary Medical Colleges

American Association of Feline Practitioners
The AAFP is like the American Medical Association for cat doctors, and has lots of good information regarding feline health

American Heartworm Society
Information about heartworm disease in dogs and cats

American Veterinary Medical Association
Pet health care information

ASPCA - Virtual Pet Behaviorist
Behavioral advice for cats, dogs and horses

Veterinary Information Network
VIN is the world's first and largest online veterinary community