MyFitnessPal's Calorie Counter and Fitness Tracker is by far the most popular weight loss app in the planet. The app has more than 300,000 reviews on iTunes and just as many reviews on the Android market. The app has everything a modern dieter needs to keep track of her weight loss progress, including a food database of over 2 million items. It also has a free barcode scanner and can work offline. Best of all, it is free.