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.
autoplay lets two chess engines play against each other fully automatically. You can use it to run your own chess engines tournament or stress test your own chess engine. It can also let two engines play remotely against each other. It support chess engines that support the XBoard protocol.