Microsoft Flight Simulator 2004: A Century of Flight, by itself, was an excellent product. But the freeware addons created for it elevated the simulator into a living, breathing aviation world. Through thousands of aircraft repaints, millions of square miles of improved terrain, and indispensable utility tools, amateur developers gave FS2004 a second life that outlasted its own retail support. For flight simulation enthusiasts, the lesson is clear: while Microsoft provided the sky, it was the freeware community that taught it how to fly.
Here are interesting freeware FS2004 addon types and where to look for gems. Fs2004 Addons Freeware
Freeware developers filled massive gaps left by Microsoft, modeling obscure regional airports and local airlines that commercial developers ignored. Microsoft Flight Simulator 2004: A Century of Flight,