The Zone diet is often used by those following the paleo diet.
While paleo tells you what to eat, zone is a system for determining how much to eat.

The diet centers on a “40:30:30” ratio of calories obtained daily from carbohydrates, proteins, and fats, respectively.

A truly awesome article that explains this concept appeared in the CrossFit Journal and can be downloaded here as a PDF.

This video from Virtual Cross Training, explains the basics of the Zone diet: