Found this amazing online multiplayer deathmatch game for Windows and Linux, which was recently released on March 2, 2007 called Alien Arena 2007. Similar to games as Quake III and Unreal Tournament but with a retro alien theme. The game features 35 maps, eleven player characters, and five game play modes to choose from. As for weapons you start out with the standard blaster and move up. The most powerful is the 'Alien Vaporizer'.
The Human Construct
It is brought to you by COR Entertainment. With great graphics and fast game play, I still can't believe that this game is completely free without any limitations.
The latest version 6.04 fixes some bugs in the earlier versions. Only drawback is the size of the file (70.7MB) which might take a while to download.