Orbiter spaceflight sim - learning orbital mechanics, modifying add-on spacecraft and learning how to program my own. http://orbit.medphys.ucl.ac.uk/

NASA flight software source code for Apollo and other spacecraft. Emulator: https://www.ibiblio.org/apollo/yaAGC.html Code: https://github.com/virtualagc/virtualagc

Probably the most realistic Space Shuttle simulator ever created (runs in FlightGear). https://wiki.flightgear.org/Space_Shuttle