Oooh, I hadn't thought about that. Like the flavored syrups used for coffees? I was thinking more on the lines of the tiny bottles of artificial flavoring used in baking or candy making. Those are generally water based with chemical additives, but have no sweetening power.
If you are using the syrups, then you will have calories. Torani syrups list their nutrition info here http://www.torani.com/create/fpsyrups.shtml
and it averages 75 calories per 2 Tbsp serving. The sugar free version has 0 calories.