Tremulous

Basic Information
Tremulous is a free and open source asymmetric team-based first-person shooter with real-time strategy elements. Being a cross-platform development project the game is available for Windows, Linux, and Mac OS X.

The game features two opposing teams: humans and aliens. Each team must attack the enemy's base and team members while defending their own base.

Tremulous can be downloaded here

Gameplay
Players can choose from 2 unique races, aliens and humans. Players on both teams are able to build working structures in-game like an RTS. These structures provide many functions, the most important being spawning. The designated builders must ensure there are spawn structures or other players will not be able to rejoin the game after death. Other structures provide automated base defence (to some degree), healing functions and much more.

Player advancement is different depending on which team you are on. As a human, players are rewarded with credits for each alien kill. These credits may be used to purchase new weapons and upgrades from the "Armoury". The alien team advances quite differently. Upon killing a human foe, the alien is able to evolve into a new class. The more kills gained the more powerful the classes available.

The overall objective behind Tremulous is to eliminate the opposing team. This is achieved by not only killing the opposing players but also removing their ability to respawn by destroying their spawn structures.

Development
Tremulous' development by the Dark Legion group commenced in early 2000 as a modification for the commercial computer game Quake III Arena. The gameplay is generally inspired by the Quake II modification Gloom, which also features alien vs human teams with distinct user classes. Although, Tremulous shares no content with Gloom or other similar themed games from around the same time.

Following the release of the Quake III Arena source code under the GPL on August 19, 2005, the developers decided to rework Tremulous into a standalone, free and open source game. Tremulous is licensed under the GPL, although it includes code from other projects that was released under other GPL-compatible licenses. Most of the game media is licensed under the CC BY-SA 2.5 Creative Commons license.

Its initial standalone version 1.0.0 was released on August 11, 2005. With the stable version 1.1.0 released on March 31, 2006 the game change its game engine to ioquake3, a modified id Tech 3 engine. In 2008, Tony J. White of the ETPub mod for Wolfenstein: Enemy Territory, contributed an administrative system and a backported client that were soon incorporated into the official Subversion repository. The latest public build version on the icculus repository is 1.2 beta "Gameplay Preview" (aka GPP), released on December 4, 2009 with the latest updates released on November 8, 2011

A Tremulous continuation from Dark Legion has its source code repository migrated to GitHub and is not under active development (last commit June 2016).