Quake 2 Map Info is a command line utility that parses a Quake 2 map file and reports how many entities and textures are being used. It is useful for spotting extra items to improve gameplay balance and/or reducing extra textures to increase map speed. Most map editors do not provide a detailed breakdown of what is inside a map. It is compatible with any Quake 2 (ID Tech 2) game engine if the map file format has not be altered. If a game has a different map format, you can modify the source code to reflect the new format.
D'Enfent Engine is a library that contains high-level classes to access OpenGL functions. It contains classes that may be very useful in game development: GUI components, sprite classes, and classes to handle mouse/keyboard events. It has a full API reference, as well as lessons to help you start working with the library.
Zaz is an arcade action puzzle game where the objective is to get rid of all incoming balls by rearranging their order. Currently it includes 20 different levels and two modes of gameplay. The engine allows for easy custom level creation with unlimited number of paths, different speeds, ball-sizes, and rules. A 3D accelerator is needed for decent gameplay.
Morris is an implementation of Nine Men's Morris that also supports several rule-variants and different board layouts (including Morabaraba). You can play against the computer, or play against another human opponent. The computer opponent learns from previous games and tries not to make the same mistake twice. This ensures continuous variation in game play, should you manage to beat the program. Furthermore, the program can give move hints and provides unlimited undo/redo.