Sweet 'N Low (Saccharin) - earliest form of sugar substitute; 300-500 times sweeter than sugar; it is not stable at high temperatures and results in a bitter taste, best used at the end of recipes; 2 teaspoons Sweet 'N Low = 1/4 cup granulated sugar.
Equal (Aspartame) - made of 2 amino acids, the building blocks of protein; 180-200 times sweeter than sugar; breaks down and loses its sweet taste when heated, best for use in cold recipes, you may use 1/4 cup Equal in place of 1/4 cup sugar; new Equal Sugar Lite is a product made especially for baking that is half sugar and half encapsulated Equal (which makes it heat stable).
Splenda (Sucralose) - made from sugar so its taste is closest to sugar; may be used teaspoon for teaspoon in place of sugar. Splenda is heat stable and may be used in all types of recipes. Also available in a sugar blend, called Splenda for Baking, which is half sugar, half Splenda.
Stevia - derived from the plant, Stevia rebaudiana, which is an herb; native to Paraguay. It can only be sold as a dietary supplement in the U.S. and therefore is not labeled as a sweetener; in its natural state it is 300 times sweeter than sugar, and even more so when processed; it is heat stable and may be used in cooking; 1/2 teaspoon of Stevia is equivalent to 2 teaspoons sugar.