fheroes2 is a recreation of Heroes of Might and Magic II game engine
scene-oriented, flexible 3D engine (C++, Python, C#, Java)
A fast, scalable, distributed game server engine/framework for C++
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
cross-platform 2d game engine
A simple WinAPI game engine