Real Mah Jongg is a social game that originated in China thousands of years ago. Four players, named after the four winds, take tiles from a wall in turn. The best tiles are made of ivory and wood; they click pleasantly when you knock them together. Computer Solitaire Mah Jongg (xmahjongg being one of the sillier examples) is nothing like that but it's fun, or it must be, since there are like 300 shareware versions available for Windows. This one is for X11 and it's free.
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.