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.
papercut is command line based rock, paper, scissors game for Unix based operating systems like GNU/Linux, BSD, etc. A user can play the game by entering their choice of rock (R or r), paper (P or p), or scissors (S or s). papercut also stores user statistics, i.e. number of wins, losses, and draws.
Ultimate Basketball Challenge is a 5 on 5 basketball game for Unix variants including Linux and FreeBSD. It aims to be fully customizable (add/edit teams, players, stats, courts, arenas, etc.). You can currently play against the basic AI. It is in constant heavy development and new features are added often.
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.