Global Virtual Airlines, serving the FlightGear community since 2010 as Atlas Virtual Airlines has expanded and gone Global! Join them in creating the best Virtual Airline experience available anywhere. Global Virtual offers over 400 routes to fly and a wide variety of aircraft to fly them in. Everything from long haul international flights to regional turboprop commuter schedules and quick hops around the Caribbean and Hawaiian islands are available. All that's needed is a pilot to fly them, and that's you! Visit their site for more details.
ATC Europe has ATC events in surprise surprise, Europe. The website gives a clean overview of their events, when and where to fly to have a real ATC guide your departure or arrival.
You can also place your ATC session on the site for others to see.
This UK website houses 2 hangars and a forum. You can register on this site to use the forum and chat with other (UK) members. The two hangars contain numerous aircraft you can install to your FlightGear setup.
There is also a main website of FlightGear. It provides quick access to most if not all related projects and important things you need to know about FlightGear. And if you like coffee, you can order a nice FlightGear mug too!
There is a HUGE wiki site about FlightGear, how to get it, how to install it, how to fly, how to land, how to contribute to the project and much much more. Be sure to search for help with a particular problem you are having with FlightGear and chances are you'll find the solution or explanation.
What would aircraft be without the proper liveries. Naked! And we do not want to see that. Therefor there is a FlightGear Livery Database, dedicated for listing all submitted (they do not arrive there automatically) liveries for the aircraft that are available in FlightGear.