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
Full featured MUD server with all the trimmings.
Game engine supporting many 3D/2D formats and graphic effects
A cell-based puzzle game featuring classic and completely new puzzles
Vivid3D is a C++ IDE/3D engine, with python scripting.
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
Multi-threaded game engine for Android
Gorgon Game Engine (GGE) is a C++ game engine that handles many tasks.
A generic 2D engine for any kind of game
Text Based Game Engine and Parser
2D Game Engine, based on OpenGL and focused on Isometric Games
Tactical, turn-based game engine, that uses OpenGL and WinAPI