Quake III Arena is the third installment of the extremely popular and successful Quake series by id software. The game was released by id software for both Windows and Linux. The Linux version of the game was maintained and sold by the now defunct Loki Entertainment. The powerful Quake III engine is the basis for many other commercial games, as id's major source of revenue is licensing their game engines. The engine allows for user-contributed modifications to be made, allowing the game to be extensible and expandable. This game is commercial software, and requires the data from a retail copy of the game to play. If you're interested in checking out the technology behind Quake III, then "emerge quake3-demo" to get the playable demo.