The most popular is definitely FlightRadar24, as per Vulcan24's link above.
However NATS (which runs the UK's Air Traffic Control Systems) introduced their own app, Airspace Explorer, a couple of years ago, which has certain advantages over its rivals. Since it was primarily designed for use by NATS own staff though, who all use iPads, there's only an iOS version of it available, with no plans to introduce a version for either Android or Windows:
https://itunes.apple.com/gb/app/nats-airspace-explorer/id1079364261?mt=8