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.
Cadaver is a simulated world of cyborgs and nature in realtime. The battlefield consists of forests, grain, water, grass, carcass (of course) and lots of other things. The game server manages the game and the rules. You start a server and connect some clients. The clients communicate with the server using a very primitive protocol. They can order cyborgs to harvest grain, attack enemies or cut forest. The game is not intended to be played by humans! Instead the idea is that you write artificial intelligence clients to beat the other artificial intelligences.
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.