Platform

Abuse (C++, Lisp, Public domain, GPL-2.0, WTFPL, mature, inactive since 2011)
CaveExpress (C, GPL-3.0, mature)
Data Storm (C, Apache-2.0, mature, inactive since 2017)
Dave Gnukem (C++, GPL-2.0, mature)
Executive Man (JavaScript, Custom, mature, inactive since 2017)
freegish (C, GPL-2.0, beta, inactive since 2017)
FreePrince (C, GPL-2.0, beta, inactive since 2011)
freeVikings (Ruby, GPL-2.0, beta)
glPortal (C++, zlib, beta)
Jazz² Resurrection (C#, JavaScript, GPL-3.0, mature)
Keen Dreams (C, Assembly, C++, GPL-2.0, mature, inactive since 2014)
Lionheart Remake (Java, GPL-3.0, beta, inactive since 2018)
Mari0 (Lua, CC-BY-NC-SA-3.0, mature, inactive since 2012)
NXEngine-evo (C++, GPL-3.0, mature)
NXEngine (C, C++, GPL-3.0, mature)
Open Surge (C, GPL-3.0, beta)
Reflection Keen (C, Lua, GPL-2.0, mature)
Smash Battle (C++, ?, beta, inactive since 2011)
Sonic Robo Blast 2 (C, C++, GPL-2.0, mature)
The Legend of Edgar (C, GPL-2.0, mature)
The Secret Chronicles of Dr. M. (C++, GPL-3.0, mature)