The Merck Veterinary Manual is a great site for researching medical issues in your pet.  It does not take the place of your Veterinarian, however.  Your first call should always be to your Veterinarian!

VeterinaryPartner.com  is here to support your veterinarian and you in the care of your companion animals by providing reliable, up-to-date animal health information from the veterinarians and experts of the Veterinary Information Network (VIN), the world's first and largest online veterinary database and community.

Canine Cancer Awareness is a tax-deductible non-profit organization whose mission is to raise awareness of the prevalence of canine cancer, its effects and the available treatment options.

Integrative Veterinary Oncology: Research and Resources page has an extensive list of cancer related link and research.

Dog Nutrition Advice is a site with a great deal of information on nutrition and diseases in dogs that can relate to nutrition and obesity in dogs.  Remember, being overweight not only diminishes your dog's quality of life, but deducts nearly two healthy years from your dog's lifespan, while increasing the risk of diseases ranging from arthritis and diabetes to heart disease and certain cancers.