Simulation

3d.city (JavaScript, GPL-3.0, mature, inactive since 2016)
Danger from the Deep (C++, GPL-2.0, beta, inactive since 2011)
FlightGear (C++, GPL-2.0, mature)
FreeTrain (C#, LGPL-2.1, beta, inactive since 2009)
Freeminer (C++, Lua, GPL-3.0, beta)
LinCity (C, GPL-2.0, mature, inactive since 2005)
LinCity-NG (C++, GPL, mature)
Micropolis (C++, Java, Python, GPL-3.0, mature)
Open Rails (C#, GPL-3.0, mature)
OpenCity (C++, GPL-2.0, beta)
OpenRCT2 (C++, GPL-3.0, beta)
OpenSkyscraper (C++, GPL-2.0, beta)
OpenTTD (C++, GPL-2.0, mature)
Our Personal Space (Ren'py, GPL-3.0, mature)
Pioneer (C++, C, Apache-2.0, GPL-3.0, mature)
SimuTrans (C++, Artistic License, mature)
micropolisJS (C++, Java, Python, GPL-3.0, mature)