FlightAware

FlightAware tracks aircraft around the world using ADS-B receivers, many of them run by volunteers, combined with data from air traffic systems and other sources. Anyone can look up a flight, follow it on a map, and see departure and arrival times, routes and aircraft details.

Pilots use it to check their own flight tracks and to watch traffic at an airport. Businesses use its data products, including an API, for things like arrival predictions, operations dashboards and alerts. People who host an ADS-B receiver can get upgraded account features.

Tracking data is for awareness, not for separation or navigation. Some aircraft can request that their flights are not shown publicly, and coverage varies by region and altitude. If you build on the API, check the data terms and how history and live data are priced.