Lose It! (Free!)

Lose It! is a great app for counting calories, with a comprehensive database of foods and activities. Set your daily calorie budget and keep track of calories for each meal. You can also record calories burned during exercise.

Weightbot ($1.99)

Track your weight in style with this easy to use (and cute!) weight-tracking robot.

C25K (Couch to 5K) ($2.99)

Once you've reached a certain fitness level, you might be ready to take it up a notch by training for a 5K run. C25K has a very user-friendly interface, and a 9-week schedule of workouts that take you from easy walk-jog workouts to the full 5K run.

RunKeeper (Free!)

If you're walking or running around the neighborhood, Runkeeper can use your phone's GPS to track how far you ran and what route you took.