OpenTTD https://www.openttd.org/
Dungeon Crawl https://crawl.develz.org/
OpenRA https://www.openra.net/
Battle for Wesnoth http://www.wesnoth.org/
FreeCiv http://freeciv.org/
Endless Sky https://endless-sky.github.io/
Brogue https://github.com/tmewett/BrogueCE
Reverse RPG https://r-rpg.com/
Open Hexagon https://github.com/vittorioromeo/SSVOpenHexagon
Star Control 2 http://sc2.sourceforge.net/
Screeps https://screeps.com/
AlephOne https://alephone.lhowon.org/
Xonotic https://xonotic.org/