I've found Kayak's app to be pretty useful + up to date for tracking flights. You can input a flight number, or search for flights based on departure and arrival airport. I haven't found any other app that reacts to delays or gate changes as quickly. I believe they get their data by aggregating a lot of individual sources (https://www.quora.com/How-does-Kayak-get-all-information-fro...)

https://itunes.apple.com/us/app/kayak-flights-hotels-cars/id...