As an Aviation enthusiast and pilot (I had to say it), I had to take the opportunity to setup a little ADS-B receiver.
"ADS-B" stands for "Automatic Dependent Surveillance–Broadcast".
Basicaly, airplanes equipped with an ADS-B Out are broadcasting informations like their speed, altitude, heading by radio on 1090MHz. In some countries and airspaces, having an ADS-B Out is mandatory.
Any computer with a radio receiver in range can read those informations.
A such received (search for RTL-SDR) is available for less than 30$, coming with its antena, so you just have to plug it on you computer USB port and use the appropriate software.
Some well known websites like FlightAware are taking great avantage of that feature to display almost real-time information about airplanes in the world.
They even encourage people to install their own ADS-B received and provide the software.
This is what I did. The instructions are provided here.
I ran the "SkyAware" software provided by FlightAware on a Raspberry Pi, on my roof, and peacefully see all the planes flying over Montréal (and I'm not the only one).