Alien Arena is a free, stand-alone deathmatch game based on source code released by id Software. 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 campaigns against bots.
The game has been free to play since its inception, and there are currently no plans to change it to a pay-to-play format. However, as of version 7.20, it features in-game advertising, in the main menu and in some maps.
Some of the new features in this release:

• Fixed bug with Nvidia 191.xx drivers causing game to crash on startup
  
• Dynamic world shadows using shadowmaps
  
• Game no longer automatically sets video settings on first run.
  
• New level, Dm-zion2k9!
  
• Splash damage triggers hit indicator.
  
• New spawn effects.