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.
Netrek is a 16-player two-dimensional graphical real-time battle simulation with a Star Trek theme. A game is divided into two teams of 8 players (or fewer), who dogfight each other and attempt to conquer each other's planets. Netrek is the probably the first video game which can accurately be described as a "sport". It has more in common with basketball than with arcade games or Quake. Its vast and expanding array of tactics and strategies allows for many different play styles; the best players are the ones who think fastest, not necessarily the ones who twitch most effectively.
Warship Battle is based on the old game of battleship, but with customization features. You can change the field size and the number of boats. Boats can be put one by one on the field or generated. Choose to strike in turn or until you miss. Three levels of difficulty are available. Preferences are remembered between sessions.