The apps I like for the iphone are SparkPeople, My Fitness Pal, Fooducate, and Nike Training. Those are the ones I am using right now and I really like them.
Fooducate is really cool because you can scan food item bar codes and they get a rating and they also give a healthier option to the item that was scanned!
I used to use LoseIt and I tried the livestrong App. My favorite is MyFitnessPal. Not only does it have the largest database of foods I have ever seen, but it allows you to see how your friends are doing in terms of staying on track. For example when someone loses a pound, it will post it on your friend wall. You can also access it from your computer, where you have many more interactive options. Try it out!