Orbiter

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/

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

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/

Orbiter In Only One Page
Offers links and files.
http://www.geocities.com/jig_2001/

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

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

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