FlightGear Project has always been free, open-source software. Distribution FG on GNU GPL license is one of the most important condition, which developers established many years ago and I don't think, it may be changed in future.
As I promised, aircraft list:
http://wiki.flightgear.org/Boeing_787-8_Dreamliner
http://wiki.flightgear.org/Airbus_A330-200_Series
http://wiki.flightgear.org/Beechcraft_B1900D
http://equipe-flightgear.forumactif.com/t835-hangar-de-la-paf-paf-team-hangar - DR400, Tecnam P92 Echo
http://wiki.flightgear.org/Douglas_C-47
http://wiki.flightgear.org/Grumman_F-14_Tomcat
http://wiki.flightgear.org/Piper_PA34-200T_Seneca_II (sadly veery basic 3d model)
http://wiki.flightgear.org/Spitfire
IAR80 (in this topic)
Cessna 337G Skymaster (in this topic)
Bonus:http://wiki.flightgear.org/Paper_airplane ^_^
FlightGear needs some customization after first run and it isn't so simple for newbies to find high quality addons. Using wiki shouldn't be ground state of using FlightGear, but it's helpful.
This topic is very general and contains almost everything and anything. Is there a need to create topic for beginners who need some help (even if they use FS)? Maybe how to turn on AA, Rembrandt?