GemRB is a portable open-source implementation of the Infinity Engine
Real Time Strategy game engine supporting early Westwood classics
Action RPG that's easy to mod
lua dev plugin for unreal engine 4 or 5
3D game engine/framework in C, with Luajit and Python bindings now
Gideros Release version
Game engine supporting many 3D/2D formats and graphic effects
Orx: Portable Game Engine
A free transportation simulator game
A modern, cross-platform Quake game engine based on FitzQuake.
Good-looking and addictive OpenSource games
Odamex is a free, cross-platform, modern client-server Doom engine
A cross-platform port of Hexen II game.
Arianne is an engine to develop multiplayer online games like Stendhal
Computer opponent for the board game Blokus.
Full featured MUD server with all the trimmings.
SDL based generic 2D game engine
Old-school 2D game engine
A game engine for console based text games
C++ lightweight cross-platform game engine