Orbiter

http://orbit.medphys.ucl.ac.uk/orbit.html

ORBITER is a free flight simulator that goes beyond the confines of Earth's atmosphere. Launch the Space Shuttle from Kennedy Space Center to deploy a satellite, rendezvous with the International Space Station or take the futuristic Delta-glider for a tour through the solar system - the choice is yours. But make no mistake - ORBITER is not a space shooter. The emphasis is firmly on realism, and the learning curve can be steep. Be prepared to invest some time and effort to brush up on your orbital mechanics background.

Links of Interest

http://en.wikipedia.org/wiki/Orbiter_(sim)
http://www.orbiterwiki.org/wiki/Main_Page
http://orbit.m6.net/Forum/default.aspx
http://www.orbithangar.com/
http://www.ibiblio.org/mscorbit/mscforum/index.php
http://moonport.org/
http://simcosmos.planetaclix.pt/