The secret sauce of games
Here are links to game with source code and also documentation about the game formats. Some of them are commercial games that have released source code and other are ones that was open source. Links to more games can be found at http://osgameclones.com.
Arx Fatalis – http://www.arkane-studios.com/uk/arx_downloads.php
Alien Vs Predator - https://github.com/miohtama/aliens-vs-predator
Age of Empires II - https://github.com/SFTtech/openage
Abuse - https://github.com/osgcc/abuse
C
C&C Red Alert -
OpenRA - Open Sourc C&C Red Alert
D
Descent – https://github.com/osgcc/descent-pc
Descent 2 – https://github.com/osgcc/descent2
Duke Nukem 3D – ftp://ftp.3drealms.com/source/duke3dsource.zip
E
Enemy-Territory – https://github.com/id-Software/Enemy-Territory
F
Freeciv – http://sourceforge.net/projects/freeciv/
FreeSpace 2 - http://svn.icculus.org/fs2open/
H
Heretic / Hexen – http://sourceforge.net/projects/heretic/
homeworld - https://github.com/aheadley/homeworld
J
Jedi Knight III: Jedi Academy - https://github.com/OpenSourcedGames/jedi_academy
Jedi Knight II: Jedi Outcast - https://github.com/PJayB/jk2src
M
Meridian 59 - https://github.com/Meridian59/Meridian59
Miner Wars 2081 - https://github.com/KeenSoftwareHouse/Miner-Wars-2081
Morrowind - https://openmw.org
N
No one lives forever - https://github.com/osgcc/no-one-lives-forever
No one lives forever 2 - https://github.com/wilkie/no-one-lives-forever-2
P
Penumbra Overture - https://github.com/FrictionalGames/PenumbraOverture
Postal - https://bitbucket.org/gopostal/postal-1-open-source/src
Prince of Persia - https://github.com/jmechner/Prince-of-Persia-Apple-II
U
Ultima 7
Ultima 8
Ultima Underworld
Q
http://www.gamers.org/dEngine/quake/spec/quake-spec34/qkmenu.htm
Quake 2 – https://github.com/id-Software/Quake-2
http://www.flipcode.com/archives/Quake_2_BSP_File_Format.shtml
Quake-III-Arena – https://github.com/id-Software/Quake-III-Arena
R
Return to Castle Wolfenstein – https://github.com/id-Software/RTCW-SP
Rise of the Triad – https://github.com/OpenSourcedGames/Rise-of-the-Triad
S
Serious Same - Serious Engine
Shadow Warrior – ftp://ftp.3drealms.com/source/shadowwarriorsource.zip
scummvm - https://github.com/scummvm/scummvm
Simcity - https://github.com/SimHacker/micropolis
Shadowgrounds - https://github.com/vayerx/shadowgrounds
T
W
Wolfenstein 3D - https://github.com/id-Software/wolf3d
X
Historical Source
https://github.com/historicalsource
OpenArthurianX6 - 2021
https://github.com/slashman/OpenArthurianX6