Xonotic

Xonotic is a free and open source arena-style first person shooter. Features 16 full weapons with each having a primary fire and a secondary fire (Unreal Tournament style). All classic modes are available, such as Deathmatch, Capture The Flag, and Clan Arena, plus other wacky ones. It aims to match Quake's gameplay with it's "easy to play, but hard to master".

Started as a fork of Nexuiz, after controversy surrounding the game's development. Game's engine is itself heavily modified from the Quake's DarkPlaces Engine.