Space

Orbiter Space Flight Simulator (Free)
http://orbit.medphys.ucl.ac.uk/

Orbiter Wiki
http://www.orbiterwiki.org/wiki/Main_Page

Kerbal Space Program
https://kerbalspaceprogram.com/en/

KSP Wiki
http://wiki.kerbalspaceprogram.com/wiki/Main_Page

KSP Orbital Mechanics 101
http://forum.kerbalspaceprogram.com/threads/15652-Orbital-Mechanics-101-...

Celestia
http://sourceforge.net/projects/celestia/

SpaceEngine
http://en.spaceengine.org/

STK
http://www.agi.com/products/stk/