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.
The Interreality project is a collection of Open Source, Free Software projects based on the Virtual Object System (VOS) platform. The goal is to develop a free and open platform for multi-user 3D virtual reality games, applications, and interactive, collaborative 3D virtual spaces. VOS is a powerful and generic network object framework designed to support a variety of high-performance applications. It is the basis for Ter'Angreal, a general-purpose browser application for entering and participating in 3D worlds.
EQEMu EverQuest Server Emulation Project is a project designed to fully emulate the EverQuest servers for use with the EverQuest(tm) client. This allows users to use their EverQuest(tm) client to connect to "unofficial" servers or to run their own EverQuest(tm) server. It is currently being developed for multiple platforms.
Euchre is a spades or bridge-like card game played with four players (3 computer players and 1 human). The game is played via a GTK+ interface and allows customization of the computer skill level, aggression level, and many of the other game parameters. There are three levels of AI skill (easy, medium, and hard). The medium and hard levels are good enough to give you a decent challenge.
libpubsub provides everything needed to set up a simple publish/subscribe environment. It allows a programmer to let several processes (not necessarily on the same host) exchange information with each other in a very easy way. It is mainly intended for use in C++ applications, but the package also includes a tool to let shell scripts use pubsub.