Ct.js is a desktop game engine that makes learning programming fun
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
Haskell game engine library for roguelike dungeon crawlers
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.
An online multiplayer turn based strategy game and board game engine.