Yacon (Polymnia sonchifolia) is a tuber grown originally in the Andes mountains. The edible tuber is sweet and crisp and can be peeled and eaten raw, boiled or baked. It tastes similar to an apple.
Aside from being very nutritious it is also low in calorie and high in inulin (fiber). The people at Seeds of Change say it is very easy to grow and offer small plants for sale. They also mentioned the tubers are hard to start from seed.
Yacon powder is made from dried yacon root. It is used as a sweetener in smoothies and can (or may) be used as a sugar substitute in some recipes. If you considering this as a sugar substitute you may want to read this article Yacon - The Root of Sweetness, which describes some experiments done by a type 1 diabetic.
Yacon Syrup is another product made from the yacon root. Dr. Weil suggests limiting this product to no more that 2 teaspoons per day because it is known to cause bloating and diarrhea. That said it is sufficiently sweet for coffee or tea and disburses in liquid much better than yacon powder.