Frisk is a networked version of the Parker Brothers' classic game of strategy. It is a faithful reproduction, and includes some computer players, an API for creating computer players, and a chat facility for talking to other players. Its design is client-server, using TCP/IP sockets for communication, and the source is mostly well organized and commented. It was tested on Linux and *BSD, but should work on most *nixes with X installed.
Xconq is a general strategy game system. It is a complete system that includes all the components: a portable engine, graphical interfaces for several platforms, multiple AIs, networking for multi-player games, and an extensive game library. The games in the library range from ancient civilizations to the far future, from simple to very complicated. Xconq's forte is turn-based strategy with overhead map views, using a hexagonally-tiled world. It includes a game design language GDL, which is a declarative language that lets you define the units, terrain, etc, then fill in tables defining their interaction. Literally hundreds of different properties and tables are available to the game designer.
XBoard is a graphical chessboard for X that can serve as a user interface to play against any of a variety of chess engines that run on your local machine, or against human or computer opponents on an Internet chess server. XBoard can also be used by itself to read and write game files in PGN (portable game notation). WinBoard is a port of XBoard to Microsoft Win32.
QuakeWorld is an Internet multi-player specific version of Quake. While the original version of Quake can be played over the Internet, many users modem users - the majority of players, had less than satisfactory play. Symptoms like excessive lag - actions actually happening much later than you did them; packet loss - the game would freeze and resume several seconds later; and various other difficulties plagued users. QuakeWorld is optimized for the average modem Internet player. It does only 1 thing, play deathmatch games over a TCP/IP network such as the Internet. It has no support for solo play, and you can't do anything with out connecting to a special server.