List

Check the game source sections for links to source code for some of these.

Underworld Engine - 1992

Ultima Underworld, Ultima Underworld II and System Shock.

Id Tech 1 - 1993

Doom, Heretic and Hexen.

Doom engine code review - 2010

Doom Iphone code review - 2010

Descent - 1995

Descent & Descent II

Build Engine - 1995

Duke Nukem 3D, Shadow Warrior and Blood.

Duke Nukem 3D Code Review - 2013

XnGine - 1995

The Terminator: Future Shock, Terminator: SkyNET and TES 2: Daggerfall.

Id Tech 2 - 1997

Quake, Quake 2

Quake Engine code review - 2009

Quake 2 Source Code Review - 2011

Sith engine - 1997

Jedi Knight and Jedi Knight: Mysteries of the Sith

Dark Engine - 1998

Thief: The Dark Project, Thief II: The Metal Age System Shock 2

The 3D Software Rendering Technology of 1998's Thief: The Dark Project - 2011

Unreal Engine 1 - 1998

Unreal, Unreal Tournament, Deus Ex and Rune.

Id Tech 3 - 1999

Quake 3 Arena

The Quake 3 Rendering Engine - 1999

Quake 3 Source Code Review - 2012

Unreal Engine 2 - 2003

Unreal II, Unreal Tournament 2004, BioShock and Thief: Deadly Shadows

Id Tech 4 - 2004

Doom III

Doom3 Source Code Review - 2012

Doom3 BFG Source Code Review - 2013

Doom3 BFG Documentation - 2013

Cry Engine 1 - 2004

Far Cry

Source engine - 2004

Unreal Engine 3 - 2006

Gears of War, Mass Effect and. Unreal Tournament 3

Cry Engine 2 - 2007

Crysis

Id Tech 5 - 2011

Rage

Cry Engine 3 - 2011

Crysis 2, Crysis 3 and Sniper: Ghost Warrior 2

Others

Jurassic Park: Trespasser CG Source Code Review - 2014

Prince Of Persia Code Review - 2013

"Another World" Code Review - 2011

Wolfenstein 3D for iPhone - 2009

The G3D Innovation Engine

http://casual-effects.com/g3d/

https://github.com/openxray/xray-16

Lumix Engine

https://github.com/nem0/lumixengine

Tombstone Engine API Documentation

http://tombstoneengine.com/docs/

WickedEngine

https://github.com/turanszkij/WickedEngine