Some pet stores (typically not big chain ones, but more like mom & pop ones) will sell live brine shrimp as feeder food for fish. I've never met a tropical fish that didn't go nuts for live brine shrimp.
However, the nutritional value for brine shrimp (well, adult brine shrimp at least) is somewhat limited. They are a nice supplement to add to you tank once in a while, but they should never make up the majority of a fish's diet. Because they are a live food, they are great for conditioning fish that you want to encourage breeding.
As NativeKeeper mentioned, newly hatched brine shrimp (baby brine shrimp, or BBS for short) is a great "first food" for young fish fry, especially if you gut load the BBS with something nutritious like 100% spirulina powder.