http://bzflag.sourceforge.net/BZFlag is a networked tank game. You can play in teams. You can join a clan and fight against others. The basic premise is "Capture the Flag" + rendering your enemies into dead polygons.
http://www.evl.uic.edu/aej/AndyBattalion.html
Battalion tagline: "Monsters, explosions, senseless destruction. You've seen the movies, you know what to do." Cute monsters, more dead polygons.