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.
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.
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.
Plasma EBG is an empire building game with an isometric view. It runs in multiple resolutions with graphics being displayed better for higher resolutions; hence, lower resolutions do not suffer from being too large, etc. In the future several viewing engines will be available, such as EBG-opengl and EBG-2d. There is a separate module called EBG-dev which features the latest AI-development, as well as a separate server module.
Widelands is an economic and military simulation game. The map is shown in parallel perspective with 3-dimensional terrain. Buildings and units are animated. The player controls a tribe. Land is claimed and held by military presence and then used to build an economy to support military expansion and confrontation. The project is a work in progress where much remains to be done; computer players and multiplayer are in very early stages of development. Some features that have already been implemented are exploration, fog of war, sound effects, background music, adjustable simulation speed, a few tutorial campaigns, translations to several languages, and a map editor.