Stevia is a plant from the sunflower family.  One of its species has very sweet leaves that are used as a sugar substitute.  It is 300 times sweeter than sugar but has zero calories, zero carbohydrates and a zero glycemic index.  Stevia leaves contain numerous all-natural nutrients that help regulate blood sugar, including chromium, magnesium, manganese, potassium, selenium, zinc, and vitamin  B3(Niacin)

Be forewarned though, Stevia has a slightly bitter aftertaste that IMO doesn’t work that well in baked goods.  Also it’s texture and volume per sweetness ration are radically different than sugar so it can’t be used a straight substitute in recipes that call for standard sugars. For instance 1/2 teaspoon of Stevia Extra powder provides the sweetness of 1 cup of sugar.  I prefer Xylitol but use Stevia to boost sweetness in items like cookies (Xylitol is slightly less sweet and way more expensive than regular sugar.)