Stevia is a sweetener and sugar substitute extracted from the leaves of the plant species Stevia rebaudiana
Stevia (My fav is Better Stevia Liquid French Vanilla)
However; it comes in powder form and other liquid flavors. It is a non-caloric sweetener that is not going to upset your system like aspartame, chemically induced non caloric sweeteners. It also has a zero glycemic response, meaning that ingesting it will not raise your blood sugar.
A little goes a long way and you can bake with it, add it to coffee or teas, plain yogurt, cereal etc...
References
https://en.wikipedia.org/wiki/Stevia