If you're ... trying to lose weight 
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Fooducate
You'll get rankings of your food choices from A (great) to D (bad), plus healthier alternative suggestions based on your weight, height, gender, and diet goals. (Check out what a perfect day of eating looks like.)
Lose It!
You can create your own weight loss plan based on your goals and track the calories you take in and burn.
If you're ... looking for fitness tracking
Fitbit
Want weekly summaries of calories burned, miles logged, and weight gained or lost? This is the app for you. (Requires device purchase.) (Here are 20 health trackers that can tell you more about yourself than you ever thought possible.)
Fitocracy
Why it works: It increases your motivation by encouraging you to share your progress on social media and offers to connect you with virtual coaches, who can further help you with your goals.
MapMyFitness
In addition to usual features (calorie-burning, workout tracking), this app syncs with multiple devices.
Nike+ Running
You'll get indoor and outdoor run-tracking, as well as audio feedback during your run about your pace and calories burned. (Requires device purchase.)
RunKeeper
Along with tracking exercise and weight loss, this popular app syncs with 70 other apps.
Runtastic
You'll find out how many calories you burned plus your distance, speed, and elevation gains while biking, running, or walking. Tracks your route on a map so you'll know where—and how far—to go.
Run with Map My Run
It tracks indoor and outdoor aerobics and resistance exercises; it also tells you how many calories you've burned.
Strava Running and Cycling
You can compare your performance to your friends' performance on social media while gathering suggestions on the best local areas to run and bike. "The addition of positive social support from social networks can promote feedback and friendly competition," Higgins says.
If you're ... looking for a challenge
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Charity Miles
Each mile you log earns you money to donate to a charity of your choice. You'll get 10 cents a mile for cycling and 25 cents a mile for running and walking.
Couch to 5K
Not surprisingly, this is for people who get very little to no physical activity. In just 3 months, the virtual coaches and training plans will prepare users to run a 5K. (Check out our easy running plan to get you to your first 5K.)
Runtastic Six-Pack: Abs Trainer
Choose from 20 different core-strengthening exercises that'll help you achieve a more toned tummy in just 10 weeks. You'll get guidance on all the exercises and the muscle groups they target.
