Flightradar24

Flightradar24 is a Swedish internet-based service that shows real-time commercial aircraft flight tracking information on a map. It includes flight tracking information, origins and destinations, flight numbers, aircraft types, positions, altitudes, headings and speeds. It can also show time-lapse replays of previous tracks and historical flight data by airline, aircraft, aircraft type, area or airport. It aggregates data from multiple sources but, outside of the United States, mostly from crowdsourced information gathering by volunteers with ADS-B receivers, and from satellite-based ADS-B receivers. 
The service is available via a web page or mobile device apps. "The Guardian" considers the site to be "authoritative".
History.
The service was founded by two Swedish aviation enthusiasts in 2006 as Flygbilligt.com and later Flygradar.nu for Northern and Central Europe. The service was opened in 2009, allowing anyone with a suitable ADS-B receiver to contribute data.
From 3 March 2020, ADS-B data collected by satellite was made available to all users. Aircraft located using satellite data are coloured blue on the map, and yellow if located by terrestrial receivers. 
The service received extensive exposure in 2010 when international media relied on it to describe the flight disruption over the north Atlantic and Europe caused by the Eyjafjallajökull volcano eruptions.
In 2014 it was used by multiple major news outlets following several high-profile crashes. The disappearance of Malaysia Airlines Flight 370, and in July 2014 after Malaysia Airlines Flight 17 was shot down over Ukraine, and in December when Indonesia AirAsia Flight 8501 went missing. Flightradar24 reported that its web traffic increased to around 50 times normal and caused some access congestion to users.
In November 2015, "The Guardian" newspaper reported that Metrojet Flight 9268 en route to Saint Petersburg from Sharm el-Sheikh International Airport had broken up in the air based on information available from Flightradar24.
Tracking.
Flightradar24 aggregates data from six sources:
Privacy.
The site blocks some ADS-B information from display for "security and privacy" purposes. For instance, the position of the Japanese Air Force One aircraft used by the Japanese emperor and prime minister was visible on the site until August 2014, when the Japanese Ministry of Defense requested that the information be blocked. This has subsequently meant that the aircraft no longer has its flight track posted online or on the site.