RetroArch is a frontend for emulators, game engines and media players.
Botcraft is a cross-platform C++ library to create bots
Game engine supporting many 3D/2D formats and graphic effects
Web powered cross-platform 3D, WebXR game engine
Java library to BiDiB (see http://www.bidib.org/)
Asphyre is a cross-platform framework for Delphi and FreePascal.
Sims 3 (and other) game package library
Android, DOS, FirefoxOS, Linux, MacOS and Windows all in one..
CactusGUI - a GUI for Pygame
A puzzle game of rules acting upon rules. Create the desired pattern.
High-level C++ framework to ease game development
Keep track of your favourite TV shows.
Extremely simple joystick rumble / vibration library
Pear3DEngine is a modern and modular 3D development framework
Yet Another Chess ENgine
Scriptable GTK based Python MUD Client