The team behind SDR# application has released a lightweight receiver software for 1090 MHz ADS-B signals that uses an RTLSDR device right away. The ouput in AVR-like format (whatever that means), available over a TCP/IP connection, can be used by most common airplane tracking software.
From my shack and using a simple vertically polarized dipole sticked against the window I could receive pings from airplanes as far as 140 km.
I also tried a properly cut biquad antenna without reflector (which is bidirectional) but did not notice big improvement over the dipole. Since the whole system is 100% digital, I miss the analogue feedback that helps identifying signal strength, not counting that transmitters move continously ... and fast!
I will not go much further into this monitoring activity, but it was interesting to notice how little security is built around airborne airplane position!