Alien Arena

AlienArena a free first-person shooter

Alien Arena (initially CodeRED: Alien Arena) is a free, stand-alone first-person shooter computer game.
 Begun by COR Entertainment in 2004, the game combines a 1950s-era sci-fi atmosphere with gameplay similar to the Quake, Doom, and Unreal Tournament series.
 Alien Arena focuses mainly on online multiplayer action, although it does contain single-player matches against bots.  Alien Arena has been released for Microsoft Windows, OS X, Linux, and FreeBSD. While the game’s content is proprietary freeware, prohibiting  commercial re-sale, the CRX engine is open source.  In 2013 the assets license was loosened, which allowed the inclusion in Linux distributions.” source: wikipedia

Built for fraggers by fraggers.
 Alien Arena is a furious frag fest with arenas ranging from the small, to the massive.
With a large built-in player base, it’s never hard to find a good match going on, at any hour of the day.
The community is friendly, as well as prolific. Dozens of maps, models, and various accessories have been created by community members to add on to the game experience.” source:

 You can download the game from the project webpage or install it directly from your distribution repository through package manager.