GemRB is a portable open-source implementation of the Infinity Engine
3D game engine/framework in C, with Luajit and Python bindings now
lua dev plugin for unreal engine 4 or 5
Game engine supporting many 3D/2D formats and graphic effects
A companion for your LOTRO activities
Orx: Portable Game Engine
Arianne is an engine to develop multiplayer online games like Stendhal
Odamex is a free, cross-platform, modern client-server Doom engine
A free 3d game for PC that does not limit the player. Fight,Build,Gain
Full featured MUD server with all the trimmings.
A game engine for console based text games
A Tile-based Game Engine written in C# mimicing retro consoles.
Old-school 2D game engine
A 16 bit style game on the 16-bit IBM XT.
The imaginary eight-bit machine
A fun realistic multiplayer free-and-open-source first-person-shooter!