RetroArch is a frontend for emulators, game engines and media players.
s&box is a modern game engine, built on Valve's Source 2
A dead simple 2D game engine for Go
Open source simulation game based on Transport Tycoon Deluxe
3D space trading and combat game
Real Time Strategy game engine supporting early Westwood classics
Descent 3 by Outrage Entertainment
A 2D rigid body physics engine for the web
A game engine with an emphasis on real-time cutting-edge solutions
Zig game engine & graphics toolkit
A free transportation simulator game
3D Engine c++
A promising chess program
A cross-platform port of Hexen II game.
GemRB is a portable open-source implementation of the Infinity Engine
Android, UIQ 3.0, Series 60 Edition 5 open-source software
Domination is a game that is a bit like the board game Risk
A MUD client for Android, iOS, Linux, Mac OS X and Windows
A very efficient tutor.