x737 project: Very nice planes for X-Plane.
Skytrek: Nice scenery files for X-Plane.
X-Plane sceneries: Very nice map of the world displaying almost all available sceneries for X-Plane.
Cepstral voices: Very nice text-to-speech voices for Windows to be used e.g. in X-plane.
Boeing 737: Tech informations about the Boeing 737.
RouteFinder: A route planner for flights.
Peter's aircraft: Wonderful payware planes for X-Plane.
SMARTCOCKPIT: Online resources about airplanes (e.g. manuals) and everything related to aviation.
Wikipedia - List of commercial aircraft accidents: To simulate in X-Plane.