CatacombGL - Windows source port of Catacomb 3D and the Catacomb Adventure series.

BStone - A source port of Blake Stone: Aliens Of Gold and Blake Stone: Planet Strike.

BetterSpades - Ace Of Spades open source implementation that runs on older machines.

Allegiance - Allegiance is RTS and FPS Space Combat game.

Zelda Classic - A game and editing tool that lets you create custom Legend of Zelda quests.

The Great Escape in C - Classic ZX Spectrum game The Great Escape in portable C.

Taisei - Fan-made, open-source clone of the Touhou series.

OpenTyrian - An open-source port of Tyrian from Turbo Pascal to C.

Open Kick-Off - Rewriting attempt of the cult football game Kick Off 2.

OpenJazz - Free, open-source version of the classic Jazz Jackrabbit™ games.

OpenClonk - Multiplayer action game where you control small and nimble humanoids.

Mr.Boom - 8 player Bomberman clone for RetroArch/Libretro.

C-Dogs SDL - Classic overhead run-and-gun game.

Barbarian - Open-source clone of Barbarian: The Ultimate Warrior.

ScummVM - Allows you to run certain classic graphical point-and-click adventure games such Maniac Mansion and Monkey Island.

ResidualVM - Game engine reimplementation that allows you to play 3D adventure games such as Grim Fandango and Escape from Monkey Island.

raw(gl) - Reimplementation of Another World.

engge - Experimental game engine for Thimbleweed Park.

TOMB5 - Tomb Raider: Chronicles disassembly translated to C source code.

San Andreas Unity - Reimplementation of GTA: San Andreas game engine in Unity.

Rigel Engine - Modern reimplementation of the classic DOS game Duke Nukem II in C++.

remc2 - Reverse engineering of Magic Carpet 2 from Assembler.

re3 - Reversed source code for GTA III and GTA: Vice City.

OpenTomb - Original Tomb Raider 1-5 experience with contemporary updates, features and additions.

OpenMB - Role-playing game engine for Taleworlds' Mount&Blade Series.

OpenLara - Classic Tomb Raider open-source engine.

OpenJK - Community effort to maintain and improve Jedi Academy and Jedi Outcast by Raven Software.

MafiaUnity - Open-source Mafia game framework.

donut - Reimplementation of The Simpsons: Hit & Run in modern C++ and modern OpenGL.

Carnage3D - Reimplementation of Grand Theft Auto.

etc.), take a look at Open Source Game Clones. If you are also interested in clones and similar games (0 A.D. Actively maintained open-source game remakes.

Redneck rampage source port