XPilot is a graphical multi-player maneuvering game with less-exciting graphics, but more speed, tactics, and insults. Some people also use it for UDP network analysis. Over the past eight years of playing and development, tons of features have been added to the game, from basic weapons like shotguns and missiles to more recent ones like phasing devices, invisibility, or hyperjump.
Tetrinet for Linux is a Linux version of the popular Windows multiplayer Tetris game. It should work on all systems having the ncurses library installed. This version runs in textmode only and requires a 50-line display. Both Tetrinet server and client functionality have been implemented.
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.
PySol is an exciting collection of more than 200 solitaire card games. Its features include a very nice look and feel, multiple cardsets and table backgrounds, unlimited undo/redo, loading/saving games, player statistics and log files, a hint system, demo games, support for user-written plugins, sound support (including samples and background music), an integrated HTML help browser, and lots of documentation.