Freeciv is a multiuser reimplementation for Unix/X of the famous Microprose game of Civilization. By default, the game is an improved Civ II, but this can be customized; modpacks for near-100% compatibility with Civ I and Civ II are included. Multiuser gameplay is real-time: in each turn, all human players move concurrently. The game is designed to remain fairly playable even on poor network connections. Freeciv can also be played on standalone machines, and its AI players are a good challenge for beginners. The source code comes with the server, two X clients, and non-X clients for MS Windows and Amiga. Freeciv is released under the GNU General Public License. It is maintained by an international team of coders and enthusiasts, and is easily one of the most fun and addictive network games out there.
Netrek is a 16-player two-dimensional graphical real-time battle simulation with a Star Trek theme. A game is divided into two teams of 8 players (or fewer), who dogfight each other and attempt to conquer each other's planets. Netrek is the probably the first video game which can accurately be described as a "sport". It has more in common with basketball than with arcade games or Quake. Its vast and expanding array of tactics and strategies allows for many different play styles; the best players are the ones who think fastest, not necessarily the ones who twitch most effectively.
Warship Battle is based on the old game of battleship, but with customization features. You can change the field size and the number of boats. Boats can be put one by one on the field or generated. Choose to strike in turn or until you miss. Three levels of difficulty are available. Preferences are remembered between sessions.
XBattleAI is an enhanced version of the Unix strategy game XBattle. XBattleAI is an elegant and abstract realtime strategy game for multiple players or a single player. It is based on the last stable version of XBattle (5.4.1), and adds the option of using computer-controlled opponents, campaigns, some bugfixes, and some miscellaneous extra features.
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.