Oy, this is a tough one. Zero- or no-calorie sweeteners include everything from aspartame (NutraSweet), sucralose (Splenda), saccharin (Sweet n' Low), and stevia leaf extract (and some others). These are marketed as replacements for sugar because they taste sweet like sugar but contain no calories. Some correlational studies have posited that no-calorie sweeteners cause people to gain weight, mostly because overweight people consume more no-calorie sweeteners than normal weight people. However, it is not clear whether the overweight people ate more no-calorie sweeteners because they were trying to lose weight, or if the no-calorie sweeteners made them overweight in the first place. The study was not randomly assigned or controlled, so we have no way of knowing which is the chicken and which is the egg. What we do know is that rats fed water sweetened with no-calorie sugar substitute eat more food than rats fed sugar-sweetened water, and tend to gain more weight. The rats seek out food to replace (and then some) the calories missing in the water.
With people, we know that even no-calorie sweeteners cause your body to release insulin. You taste something sweet, and as a result your body releases insulin to absorb the anticipated sugar. When the sugar does not come (because these sweeteners contain no calories), the insulin surge instead causes your blood sugar levels to drop, making you hungry. You may find that if you're not hungry, and you drink a diet pop, an hour later or so you'll definitely be hungry. Your metabolism is not fooled by these sweeteners!
The lesson here is to consume no-calories sweeteners sparingly, and it may be best to eat them with other things rather than alone. Then you will avoid the blood sugar drops associated with the no-calorie sweeteners. I like to use them to substitute for 50% of the sugar in a recipe, which lowers the calories but does not fake-out your metabolism like the no-calorie sweetener alone - your body still gets some sugar instead of a blood sugar plunge. Stay tuned for more research in this area, and like anything else, moderation is key.