Xonitic is a free and fast first person shooter based on the Darkplaces game engine, which itself is a modified version of the Quake engine. It is available for Windows, OS X, and Linux. The game features a large number of large and interesting maps, which look eerily similar to those in Quake 3, but are unique in their own ways.
The game's graphics are similar to those of commercial games available in around 2006-2007. Gameplay in itself is also inspired by different games including Unreal Tournament and Quake, with the multi-player element being an important part. Xonotic is incredibly well-made, considering the fact that it is open source and based on a decade old game engine.
Developer: Xonotic Version: 0.5 Genre: First Person Shooter License: Freeware Platform: Multi-platform Size: 943 MB Download link: http://www.xonotic.org/download/
Take a look at the gameplay video and tutorial before you decide to download.