fheroes2 is a recreation of Heroes of Might and Magic II game engine
Open source clone of the Age of Empires II engine
Open 3D Engine (O3DE) is an Apache 2.0-licensed multi-platform 3D
Opensource crossplatform Fallout 2 game engine writen in C++ and SDL
Spring is a free RTS engine developed for Windows, Linux and Mac OS X.
A Simple Game Engine