Veterinary organizations:

American College of Veterinary Internal Medicine:
http://www.acvim.org/

American College of Veterinary Emergency and Critical Care:
http://acvecc.org/

American Veterinary Medical Association:
http://www.avma.org/

The American Animal Hospital Association:
http://www.aahanet.org/

Client information:

National Canine Cancer Foundation
http://www.wearethecure.org/

ACVIM Fact sheets: Diseases and symptoms
http://www.acvim.org/websites/acvim/index.php?p=62

Heartworm disease:
http://www.heartwormsociety.org/
http://www.Knowheartworms.org

General information on heart disease and pet health:
http://www.vetmed.wsu.edu/clientED

Genetic testing for Hypertrophic Cardiomyopathy:
http://www.vetmed.wsu.edu/clientED/hcm.aspx

Cavalier King Charles Spaniel:
http://www.cavalierhealthfoundation.com/
http://www.cavalierhealth.org/

Boxers:
http://www.americanboxerclub.org/