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.
GNU Robots is a game/diversion where you construct a program for a little robot, then watch him explore a world. The world is filled with baddies that can hurt you, objects that you can bump into, and food that you can eat. The goal of the game is to collect as many prizes as possible before you are killed by a baddie or you run out of energy.
Obsidian is an extensible virtual world system with a fullblown multiplayer client-server architecture. It is not a VRML2 world system, though there are plans to include VRML2 as a world data format. Note that active development on this system stopped a while ago, though the code is still available for the curious. The code may not even compile on current Linux or Irix systems.
The Flight Gear Flight Simulator project is a free, open-source, multi-platform, cooperative flight sim development project. Source code for the entire project is available and licensed under the GPL. The Flight Gear project is working to create a sophisticated flight simulator framework for the development and pursuit of interesting flight simulator ideas. We are developing a good basic sim that can be expanded and improved upon by anyone interested in contributing.
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.