BZFlag is a free online multiplayer 3D tank battle game. The name
originates from "Battle Zone Capture The Flag". It runs on Windows,
Mac OSX, Linux, BSD, and other platforms. It was one of the most
popular games ever on Silicon Graphics machines and continues to
be developed and improved to this day.

At its heart, the game is a 3D first person tank simulation where
opposing teams battle for dominance.

WWW: http://bzflag.org/