Ecere Chess is a very simple, but fun 3D chess game with AI and network support. It will run on any platform supported by the Ecere SDK (currently Windows and Linux). It is a good opponent with whom to practice your chess skills, as although it is not very smart, it will rapidly take advantage of your mistakes since it sees all possibilities two moves ahead.
The Python/Tk Empire Interface (PTkEI) enables you to connect to empire 4.x.x (Wolfpack) servers. Empire is a real-time war game with long tradition. You can find out about Empire and currently running servers and other clients at its homepage. This client is an example for a truly portable cross-platform GUI, known to run on Unix, X11, Win32 and Mac. Additionally you do not give up any command line power as a player, but you have to to learn the empire commands to make use of this GUI client.
Kalyp is a fantasy-based roguelike game written in Java. Its design is based on traditions set by popular roguelike games, especially Nethack. It is single player and runs in a text terminal. The project focuses mostly on the Artificial Intelligence engine for the non-player characters. Most of the game design will be done in scripting, as the core engine written in Java will handle user interface, rulesets, basic actions, and try to hold it all together.
G2F3 (Generic Game Framework in eiFFel) aims to simplify the development of two player zero sum strategy games by separating game specific code from reusable generic code. So far, abstract board games are best supported. This software is written using GNU SmartEiffel. The package comes with six example games.