Thai fish sauce, or fermented fish sauce, is a staple throughout southeast Asia (not just Thailand). It makes a great substitute for soy sauce. I use it instead of soy sauce now because it’s so much better for you. Since the fish is allowed to ferment for several weeks, you end up with a highly nutritious condiment.
Thai fish sauce can also be added to soups or stews, or anytime you want more “umamai” flavor in a dish. I use it in everything from beef stew to chili to barbecue sauce — it really boosts the flavor. Fermented fish sauce is also used in making ketchup.