Orbiter - A free Space Flight Simulator

AVSIM File Library - File repository for Addons

Vinka's Orbiter Fan Site - Home of 'Spacecraft.dll' and 'Multistage.dll'

Meadville Space Center - Project Gemini and Mercury plus other Addons

Dan's Orbiter Page - The essential 'Orbitersound' can be found here

Duncan's Orbiter Guide - MFDs, addons and tutorials (excellent Mars tutorial)

FrancisDrakex Orbiter Page - Shuttle addons and tutorials (see his Shuttle landing tutorial)

McDope's Orbiter Mods - addons, including those of the Russian Space Program

Project Alpha - Highly detailed ISS replacement

Orbiter Tutorials - An incredibly useful collection of Orbiter tutorials

Orbiter Forums:
Orbiter Web Forums
Addons
Orbiter SDK
Space Maths and Physics