BZFlag is a 3D (OpenGL) multi-player tank battle game that allows users to play against each other in a networked environment.
The latest version runs on Windows 95/98/NT/2000, Irix 5.3 or higher, Linux for i386 and PPC, and Solaris. All versions are compatible, meaning users on different platforms can play together. To play you have to connect to a server; there are several on the internet or you can start one locally.
Think capture the flag, but with tanks. It's based around an old SGI game called Battlezone (BZ.. you see). Basically the idea is you drive around in a tank, and you have to shoot other tanks. Along the way you can pick up flags that change the game somewhat - some make you go faster, some slower, some make you invisible or able to drive through buildings or change your gun into a laser etc...
Check out http://www.bzflag.org.
Check out the Wlug BzFlagCompetition!
Point Blank was (IMNSHO) a lot more fun to play - there were three different tanks and an aeroplane which you could use, along with different types of missiles.
The terrain also had hills.
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