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.
http://www.medphys.ucl.ac.uk/~martins/orbit/orbit.html

Space Shuttle Scenarios

Includes custom made scenarios reproducing the STS-112 mission and based upon real life telemetry data sets.
http://home.t-online.de/home/strambach/orbitersim/

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.
http://orbiter.dansteph.com/

Project Alpha

An ongoing project to simulate the International Space Station Alpha.
http://www.f-a-d.co.uk/

Orbiter Mars

Includes concise tutorials that explain the procedures and the planning stage of interplanetary missions.
http://www.orbitermars.co.uk/

Orbital Mods

An addon repository focused in the Russian space flight program. It also features a database of known bugs in the simulator.
http://www.orbitmods.com/

Saturn's Moons

Includes the major moons of Jupiter and Saturn, missing in the default Orbiter package.
http://www.geocities.com/fastheinz39/Saturns_Moons.html

Orbit Hangar Mods

A general repository of contributions submitted by third party add-on developers.
http://www.orbithangar.com/

Orbiter In Only One Page

Offers links and files.
http://www.geocities.com/jig_2001