Stride (formerly Xenko), a free and open-source cross-platform C# game
The Tiny JavaScript Game Engine That Can!
Game engine supporting many 3D/2D formats and graphic effects
Vivid is a modern C++ 3D engine using OpenGL4+
3D C++ Game Engine - yet another open source game engine
Open-source, cross-platform, C++ game engine for creating 2D/3D games
TurboSphere is an easy to learn and powerful JS based game engine.
Game engine supporting many 3D/2D formats and graphic effects
Zen3D is a 3D engine and editor for creating games