One of the Low Carb Diets, though really more focused on "bad carbs" like starches. Aims for balance of 40% carbohydrate, 30% protein, and 30% fat (% of calories, not weight).
Assemble meal from "blocks": A mini-block of protein is equal to 7 grams of protein, a mini block of carbohydrate is equal to 9 grams, a mini-block of fat is equal to 1.5 grams (or 3 grams when Zoning labels or when your protein source is completely fat free). To be in the Zone, you need to have your meals balanced in a 1-1-1 ratio, one mini-block of each macronutrient (protein, fat, carb) is equal to one block. To make a three block meal, you can put together three blocks any way you want, as long as it's balanced.