OTC is a 2D platform game for multiple players. It has a (more or less) realistic physics system and allows the use of various weapons. It will also include a editor for creating and editing levels.
A prototype for a 3D Goban display, full featured SGF editor, client for IGS-PandaNet and interface for GNU Go. glGo is written in C++ and based on wxWindows, OpenGL and SDL.
glGo is available for Linux and Windows.
PyCarge is a cross-platform opensource console style RPG engine written in Python. It has a built-in character and map editor. RPGMaker-like macros ~and~ full scripting cabilities are included. Allows custom character and tileset sizes.
Orange 3D, a game engine distributed under the zlib/libpng license, is an all in one solution, with IDE, level editor, 2d painter, 3d modeler, and possibly audio editing software all in one clean package, for free!