I don't know how many people here are into all things Raspberry Pi, but I found an interesting topic the other day, that might be of interest to some people here.
Apparently teaming the Raspberry Pi with some brands of USB DVB-T receivers, it is possible to receive ADS-B.
For detail see here :
Raspberry Pi ADS-B
You can even make your own antenna :
Coaxial Collinear Antenna for ADS-B