A mounting body of evidence suggests that fish oil (which contains omega-3 fatty acids) has numerous health benefits for people. And an article in the Journal of the American Veterinary Medical Association (JAVMA) suggests that fish oil and omega-3s may be good for pets, too.
Research suggests that omega-3s may: help to support the kidneys and heart, enhance skin and coat quality, reduce symptoms of allergies, reduce the prevalence of auto-immune disease, prevent and treat arthritis, and support neurological development of juvenile animals. Other sources suggest that omega-3s may help to prevent cancer.
Before you dash to the store to buy omega-3s for your pet, I should point out that the key word in the above paragraph is may. More research is necessary to determine optimum doses and blends of omega-3s.
Talk to your vet before supplementing your pets diet with fish oil, and be sure to use a product designed for cats or dogs. And remember, fish oil is oil, and oil can be fattening. In this case, too much of a good thing could contribute to a weight problem.
For reference, the paper cited in this post is J Am Vet Med Assoc 2007;231:1657-1661