The Stevia plant is a natural alternative to sugar.  It is native to South America, and the plant has been used safely by the Guarani Indians in Paraguay for hundreds of years to sweeten tea and other foods.  It is best known for its sweet taste and use as a sugar substitute, but has other properties too.  In South America, it is used to treat Type II Diabetes, because it is said to have stabilizing effects on blood sugar.  It also has antibacterial effects.
It has no calories despite its sweet taste.   It is much sweeter (250-300 times) than sugar, much the same as the artificial sweeteners.  Stevia is becoming more common in commercial beverages as a natural low-cal alternative.  It can be used in baking but can have a bitter taste that can take some getting used to, but with practice baked goods made with stevia taste great and people will not know the difference if you served them at a party! It can be purchased in both powder or liquid form in some grocery stores and almost all health food stores.