I think Lose It! is pretty great. It tracks by day and week. Based on weight, height, gender, how much you want to lose and how quickly, it determines your daily calorie requirements. You can add exercise each day. At the end of the day, it tells you whether you're over, have met, or you're under your daily total. It's cumulative throughout the week - so if you're over by 100 calories on Monday, but under by 100 on Tuesday, the weekly total for those two days makes you even.
A few of the foods listed are incorrect (a cup of cheerios in the app is 133 calories, versus the 100 listed on the box). It's an odd thing to be off. But for the most part I like the app. It's the only one I know of that tracks by the week - so you're bad one day, you can make up for it the next. There may be others, it's just the only one I know of. User friendly, too.