Here are a few of mine:
MyFitnessPal – an easy to use food journal. It contains over 1,000,000 food items including foods from your favorite restaurants. If you are counting calories, this may be the way to go.
NikePlus Running – This is a great app to help you map out any runs that you want to make, it narrows down the mileage for you. Also it helps you stay on pace, giving you encouragement through your entire run.
ChallengeLoop – Offers tons of health & wellness challenges, not just fitness, and gives you the possibility of winning prizes for participating too!
Nike Training – This is a free app with TONS of free workouts. The workouts include videos to keep you on track and also breakdowns of each exercise. This is one of the best apps out there in my opinion, especially if you prefer to work out at home rather than at a gym or in a class.