Did you know that for less than 7 Euro you can buy a digital TV USB stick that can be used as a low cost ADS-B receiver?
I got this "RTL2832U+R820T DVB-T USB Digital TV Tuner" from China and it really works brilliant.
Automatic Dependent Surveillance–Broadcast (ADS–B) is a surveillance technology in which an aircraft determines its position via satellite navigation
or other sensors and periodically broadcasts it, enabling it to be tracked. The information can be received by air traffic control ground stations as a replacement for secondary surveillance radar, as no interrogation signal is needed from the ground. It can also be received by other aircraft to provide situational awareness and allow self-separation. ADS–B is "automatic" in that it requires no pilot or external input. It is "dependent" in that it depends on data from the aircraft's navigation system. ADS-B-equipped aircraft and vehicles exchange information on one of two frequencies: 978 MHz or 1090 MHz. Mode A/C and S transponders, as well as Traffic Collision and Avoidance Systems (TCAS), use 1090 MHz. ADS-B extends the message elements of Mode S, adding information about the aircraft and its position. This extended squitter is known as 1090ES. An international technical advisory committee chose 1090ES as the worldwide standard for ADS-B.