My homemade recipe calls for fish oil. The EPA and DHA in fish oil can do many good things for cats, including:
Improved heart function, reduction of triglycerides, etc.
Healthier skin and coat
Reduction of inflammation.
It is important to get the good stuff, though. Fish oil in and of itself is a meaningless term health-wise. EPA/DHA are the elements in fish oil that give the benefit to humans and pets. In cheaper fish oil products, there is very little EPA/DHA, and a whole lot of filler crap that could actually be very bad for your cat. Also, I would avoid bottled fish oil, salmon oil, etc. Fish oil goes bad VERY quickly. Even kept in the freezer, it just doesn't stay good long enough. Go with the capsules and then dissolve in some warm water, then throw the water, oil and gelatin capsule into the food. The capsule gives the teeth a workout and is good for your kitty!