UCI chess engine
UZDoom is a feature centric port for all Doom engine games
A Minecraft mod designed to improve frame rates
3D Engine c++
Game engine supporting many 3D/2D formats and graphic effects
Skript is a Bukkit plugin which allows server admins
An open source flight dynamics & control software library
GemRB is a portable open-source implementation of the Infinity Engine
A .NET ascii/ansi console engine written in C# for MonoGame and XNA
RTSish Game Engine
Go bindings for raylib, a simple and easy-to-use library
Play Fighting Fantasy fan stories on this simple engine
A modern, cross-platform Quake game engine based on FitzQuake.
An isometric arcade-style game of skill, puzzles and exploration
Good-looking and addictive OpenSource games
Orx: Data-Driven Game Engine
A free transportation simulator game
A WIP of a very cool game engine
0 A.D. is a free, open-source, cross-platform real-time strategy game.
Chess engine written in Pascal with an UCI interface.
Arianne is an engine to develop multiplayer online games like Stendhal