fheroes2 is a recreation of Heroes of Might and Magic II game engine
scene-oriented, flexible 3D engine (C++, Python, C#, Java)
Pycraft is an open-source, OpenGL based game made in Python
GemRB is a portable open-source implementation of the Infinity Engine
A cell-based puzzle game featuring classic and completely new puzzles
Full featured MUD server with all the trimmings.
Game engine supporting many 3D/2D formats and graphic effects
A game engine for console based text games
Go 3D Game Engine
DOOM / Heretic / Hexen source port with enhanced graphics and UI
Real-time RPG for PC/tablet/phone; multiple quests and random dungeons
Gorgon Game Engine (GGE) is a C++ game engine that handles many tasks.
A generic 2D engine for any kind of game
2D Game Engine, based on OpenGL and focused on Isometric Games
Open source, moddable game engine for the "Polygon Love 2" game
cross-platform 2d game engine
Pig2D is a game engine for Java that uses a sprite-based interface.
Gomoku Game: Playing with Computer