BZFlag is a 3D, multiplayer, tank battle zone, capture the flag game that pits players against each other in a networked environment. It runs on Windows 95/98/NT/2000, Linux, MacOS 10.x, Irix, Solaris, and others. An OpenGL accelerator is highly recommended, but it is playable with 3D in software.
Cadaver is a simulated world of cyborgs and nature in realtime. The battlefield consists of forests, grain, water, grass, carcass (of course) and lots of other things. The game server manages the game and the rules. You start a server and connect some clients. The clients communicate with the server using a very primitive protocol. They can order cyborgs to harvest grain, attack enemies or cut forest. The game is not intended to be played by humans! Instead the idea is that you write artificial intelligence clients to beat the other artificial intelligences.
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.
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.