Godot is a powerful cross-platform game engine
Open source voxel game engine with support for survival and crafting
scene-oriented, flexible 3D engine (C++, Python, C#, Java)
The game engine for everyone
New Godot bindings for Swift
Urho3D Game engine
Game engine to build text adventures.
GemRB is a portable open-source implementation of the Infinity Engine
Orx: Portable Game Engine
Cross-platform 2D game engine (+ full OpenGL, +full OpenGL ES )
Open source reimplementation of GTA San Andreas game engine in Unity
2D Game Engine
A cross platform Lua based game engine
Cross-platform 2D and 3D game engine
Android OpenGL Game Engine
Real-time RPG for PC/tablet/phone; multiple quests and random dungeons
Multi-threaded game engine for Android
online 4X browser game engine
Cross platform 2D/3D game engine written in C++11 & OpenGL ES 2/3
Egret is a brand new open mobile game and application engine
2D physics engine for games
Visual Novel engine, editor and game development.