There is My Fitness Pal, which I'm familiar with but aren't using... its users boast of the ability to build comradery with friends as you log in your meals, etc. Your friends can cheer you on or nag you when you've forgotten to log in a meal.
My app of choice for an iPhone dieting tool is CALORIE COUNTER. It's free (most importantly) & it has all the bells & whistles I'd be interested in. It's easy... they ask you to put in beginning weight. And height. Then they give you a goal weight (albeit lofty). Then they figure out a max amt of calories in order to lose weight & reach said goal.
Then, the app allows you to add EACH AND EVERY SINGLE food you eat. You can do this by either SCANNING the bar code of the food item, by searching for it, or by adding it.
At anytime you can check daily nutrition total (which gives cholesterol, sodium, etc) & there are even flow charts for progress. As the day progresses, it lets you know how many more calories you're allowed, etc.
I sound like I'm a paid advertiser, but I promise I'm not... I'm just a happy user!