I also vote for LoseIt. It's a free app and it helps you track your calories. If you're counting calories, you just log it and then once you know how many calories something is, you don't have to keep looking it up. You can set up how much you want to lose per week and it will take your exercise calories burned into account as well so don't be afraid if the number it recommends you eat is too low.
It's mainly used for bicylcing but can also be used for walking/running. It's an app that uses GPS and maps the ride you are doing. It then times you. If you do the same route again, it compares it to your previous time to see if you improved or not.
Ok so I've been using MyFitnessPal and not sure I like it. I do like the little feature when you complete an entry it says "If every day were like today you'd weigh xxxlbs in 5 weeks! But it's hard to find a good calorie count for any food and I like to measure in oz and grams and it's not working for me. Switching to LoseIt for now.
I also found a cool hypnosis app that I've been listening to before bed time which helps me relax and focus on the positive.
I've downloaded iSmoothRun as well which is a GPS pedometer, it was $4.99 but very accurate and worth it I think.
I'll post here if I add any more apps. My newly acquired iphone is playing a big role in my wellness!