Simviation is a massive resource for Freeware addons for Microsoft Flight simulator series, Lockheed Martin Prepar3D (P3D), FSX, FS2004, Combat Flight Simulator and other PC Flight Simulation products like Xplane & Flightgear! Free aircraft, more planes, addons, more scenery and airports. High quality freeware aircraft addons. Expansion packs for flight simulator. You are sure to find that elusive aircraft or scenery add-on.
There is also a number of Forums for the flight Simulator enthusiast – a great place to visit if you need a flight sim poser answered.