HoloATC

This app show live air traffic from a number of airports all around the world. Initially you are asked select an airport, then select a place for the airport center. Point the phone a bit forward towards the floor, then tap the screen if the airport center is where you like it. Then you can watch air traffic. More options are in the options menu (use long press to access that)

You can also see aircraft pictures by pointing at a plane for 3 seconds, and select a single airplane to follow. This will be displayed in all devices watching this airport.