AvsimProbably the biggest add-on library. A must see!
FlightSim.comAnother big library, a bit unavailable unfortunately...
SimviationGigabytes of quality freeware downloads & add-ons for Microsoft Flight Simulator
FS CornerOne of the biggest MS-Flight Simulator Sites in the World. Over 10,000 Add-Ons for MS Flight Simulator. Aircraft, Panels, Sounds, Airports, Scenery, Tools etc.. All Downloads are freeware
FS nordicFlight Simulator Nordic is the largest Scandinavian civil flight simulation website dedicated for Flight Simulator 2004
Project AI (freeware)Make your AI traffic as real as it gets
PlaneSimulationNice Canadian site offering a wide range of quality add-ons from their 10,000+ download library. Plus unique scenery and news, tools and mini-tutorials. Free or small fee login options
FSimCafe.comProvides free access to some of the best aircraft, scenery and tools downloads for PC Flight Simulators, mainly FS2004. It features a selection of the latest files with a growing collection of the best files always available