The open-source Roblox & UEFN alternative
3D Game engine with editor
Eva.js is a front-end game engine for creating interactive projects
Godot Minimal Theme
Stride (formerly Xenko), a free and open-source cross-platform C# game
PyCAPGE - Python Classic Adventure Point and Click Game Engine
Full featured MUD server with all the trimmings.
Spring is a free RTS engine developed for Windows, Linux and Mac OS X.
Paintown is a 2d fighting game engine.
Gorgon Game Engine (GGE) is a C++ game engine that handles many tasks.