Automatic dependent surveillance—broadcast (ADS–B) is a surveillance technology in which an aircraft determines its position via satellite navigation and periodically broadcasts it, enabling it to be tracked.
It’s really easy and fun to see what planes are flying near you with a software-defined-radio dongle and dump1090.
After installing dump1090, you can see nearby aircraft information like the flight number, altitude, lat/lon, and speed. Here’s what I saw one afternoon near Memphis, TN:
./dump1090 --net --interactive
After just a few seconds on starting dump1090, planes start to appear. To view this information on a map, launch a browser and go to localhost port 8080.
It even provides links to FlightAware, so you can see even more information about a particular flight.