With a simple RTl-SDR it's possible to receive flight "beacons" which a commercial aircraft emits. There is software which in combination with an DVB-t receiver can show the planes flying in the area.
1. Connect your DVB-T receiver 2. git clone firstname.lastname@example.org:MalcolmRobb/dump1090.git 3. Run `make` (note that you need to have rtl-sdr installed)
Note: On Arch use the dump1090-git package from the AUR
1. Run ./dump1090 --interactive --net 2. Open http://localhost:8080/ in your browser to see an interactive google map