Tremulous is a free, open-source first-person shooter game for Windows, Linux, and Mac OS X. It combines elements of team-based arena combat with real-time strategy. Two teams, aliens and humans, battle for control of the map. Players on each team can choose from several classes, each with unique abilities and weapons. The game also features a built-in modding system, allowing players to create their own custom game modes and maps. Tremulous is easy to learn but difficult to master, offering hours of intense and addictive gameplay.
Discontinued No further updates since November 2013: http://assault.cubers.net/ (like Alien Arena, also noted to be discontinued).
Discontinued No more updates since January 2013: http://sauerbraten.org/. 2013, like Alien Arena, warned discontinued.
Discontinued development is not stopped, but the latest version, 0.8.8, was released in February 2012.
Discontinued The last update was published on September 6, 2013.
Discontinued Nexuiz is no longer developed. Its successor is https://alternativeapp.info/software/xonotic/, but the latest version of Nexuiz Classic, 2.5.2, released in 2009, can still be downloaded from the official website.
Discontinued development has been discontinued, as stated at https://www.warsow.net/forum/thread/17216/1. The latest version, 2.1, released in March 2016, can still be downloaded from the official website.