For the last few months, I've been tracking my weight, food, exercise and in particular, running, across three different applications. They all have API's linking to each other to make sure all the data is saved, but I still have to use each app separately to log all of this wonderful stuff.
MyFitnessPal has been used by me to log all of my food (calories, fats, carbs etc). This then sends the calorie data to Fitbit.
Fitbit is my main fitness tracker, as I've been through three separate fitness trackers from them. I started with the Fitbit Ultra (kept it in your pocket), the Fitbit Flex (the first band from Fitbit) and now the Fitbit Charge HR (also a band, with heart rate and a screen you can read). All my exercise data is kept in here, including all the weight logging. I'm actually in the process of trying to bulk up and gain muscle mass which is why my weight is increasing after successive weight-loss (25kgs!).
Runkeeper is how I track my running; splits, climb, time, GPS from my phone to log the actual route.
Fitbit has finally updated their food database to include the UK, meaning I can stop logging in MyFitnessPal. They are slowly managing to integrate a lot of the Runkeeper type metrics, so it looks like I'll be able to dump another app soon. Everything in one app (Fitbit) is much preferred to using several apps. Dashboard below from a short run yesterday :)