Unknown Horizons, RTS city builder:
http://www.unknown-horizons.org/
Dungeon Keeper clone:
http://opendungeons.sourceforge.net/
Steel Storm:
http://one.steel-storm.com/
Mario clone:
http://supertux.lethargik.org/
Settlers 2 clone:
https://wl.widelands.org/
Mario Kart clone:
http://supertuxkart.sourceforge.net/
Flight sim:
http://www.flightgear.org/
Tank FPS:
http://bzflag.org/
Space sim:
http://www.oolite.org/
Urban Terror - fps:
http://www.urbanterror.info/home/
Spring RTS engine:
https://springrts.com/
Aleph One - Marathon series clone:
http://marathon.sourceforge.net/
8bit 2d multiplayer shmup:
http://www.bitfighter.org/
Roller coaster side scroller:
http://www.stuff-o-matic.com/asgp/
Gang Garrison 2 - 2d "team fortress 2":
http://www.ganggarrison.com/
Liero clone - 2d shmup:
http://www.openlierox.net/
Neverball - action puzzle game:
http://neverball.org/
Smash battle - mario based 2d shmup:
http://sourceforge.net/projects/smashbattle/
OpenJK - Jedi Knights games:
https://github.com/JACoders/OpenJK
Open Clonk - multiplayer action side scoller:
http://www.openclonk.org/
World of Padman - childish fps:
http://worldofpadman.net/website/
2d shmup:
https://www.teeworlds.com/
2d platformer puzzle:
http://www.oletus.fi/static/whichwayisup/
2d platformer puzzle:
http://meandmyshadow.sourceforge.net/
Top down shmup:
http://www.bloodmasters.com/
Project: Starfighter:
http://starfighter.nongnu.org/
Infiniminer clone:
http://www.minetest.net/download
Syndicate clone:
http://freesynd.sourceforge.net/
Scumm engine clone:
http://scummvm.org/
Armegetron Advanced:
http://armagetronad.org/
The Ur-Quan Masters, now available in most distro repos:
http://sc2.sourceforge.net/
Nethack - roguelike game:
http://www.nethack.org/common/index.html
https://nethackwiki.com/wiki/Graphical_user_interface
> Here are some more libre roguelikes:
http://www.roguebasin.com/index.php?title=Category:Open_source
Freeablo:
http://freeablo.org/
Endless Sky - GPL Escape Velocity clone:
https://endless-sky.github.io/
OpenRCT2 - RCT2 engine replacement:
https://github.com/IntelOrca/OpenRCT2
Falltergeist is a free software remake of Fallout 2's engine:
https://github.com/falltergeist/falltergeist
OpenMW - Libre Morrowind engine with open construction set:
http://openmw.org/