Orbiter - A Free Space Flight Simulator:A realistic 3D flight simulator that uses proper physics for the modelling of planetary motion, gravitational fields, free space, and atmospheric flight.
Dan's Orbiter page: Find the module plug-in that enables sound in the simulation. Includes a support forum for frequently asked questions and general discussion.
Orbit Hangar Mods: A general repository of contributions submitted by third party add-on developers.
Orbital Mods: An addon repository focused in the Russian space flight program. It also features a database of known bugs in the simulator.
Orbiter In Only One Page: Offers links and files.
Orbiter Mars: Includes concise tutorials that explain the procedures and the planning stage of interplanetary missions.
Project Alpha: An ongoing project to simulate the International Space Station Alpha.
Saturn's Moons: Includes the major moons of Jupiter and Saturn, missing in the default Orbiter package.