Chris Hall has a helpful post on CrumpleItUp that lists eleven iPhone applications that put a "dietician in your pocket" and help track calories and more advanced analyic tools. Here's Hall's list, which is organized by cheapest to most expensive app:

1. Lose It! (Free) Distinguishing feature: Set daily calorie budgets and track your progress.
2. Sensei for Weight Loss (Free to Members) Distinguishing feature: Developed by doctors, dieticians, and psychologists.
3. CalorieCheck ($.99) Distinguishing feature: Info from the United States Department of Agriculture for 7,000+ foods.
4. Calorie Pad ($1.99) Distinguishing feature: Track caloric intake from 6,000+ products and menu items from major restaurants.
5. Calorie Abacus ($1.99) Distinguishing feature: Answers question, "If I am allowed to eat 1,000 calories, what are my options?"
6. LIVESTRONG Calorie Tracker ($2.99) Distinguishing feature: Most popular iPhone calorie tracking app.
7. Nutrition Menue - Calorie Counter ($2.99) Distinguishing feature: Nutritional info for 79,000+ food and restaurant items.
8. Fast Food Calorie Counter ($2.99) Distinguishing feature: Focuses on fast food.
9. Food Diary Calorie Counter ($2.99) Distinguishing feature: Track 90 days worth of daily food intake, while calculating your BMI.
10. Calorie Track ($2.99) Distinguishing feature: Includes data from the United States Department of Agriculture.
11. Calorie Minder ($3.99) Distinguishing feature: Access to over 7,000 foods and 32 kinds of exercise.

Check out Hall's full post with longer descriptions here.
Also, follow Hall on Twitter at @hallicious