UFO: Alien Invasion

UFO: Alien Invasion is an open source strategy video game in which the player fights aliens that are trying to take control of the Earth. The game is heavily influenced by the X-COM series (mostly by UFO: Enemy Unknown). It is based on a modified id Tech 2 engine, and runs on Linux, Microsoft Windows, and Mac OS X for both PPC and Intel Macs.

UFO:AI has been nominated for "Best project for Gamers" in the Sourceforge 2007 and 2008 Community Choice Awards and was positively noted by Linux Journal.

Overview
UFO: Alien Invasion is a squad-based tactical strategy game in the tradition of the classic X-COM series of games (the player controls a secret organisation charged with defending Earth from a brutal alien enemy). The game aims to combine military realism with hard science-fiction and the weirdness of an alien invasion and the turn-based system is supposed to give the player pin-point control of his/her squad while maintaining a sense of pace and danger.

The game takes a lot of inspiration from the X-COM series by Mythos Games and MicroProse. However, it's neither a sequel nor a remake of any X-COM or other commercial title: "What we as a team wanted to make is a brand new experience that tries to surpass the quality of games from 1992, rather than simply recreate them with flashier graphics."

Most aspects of the game are still in an evolutionary development, but it should be considered a fully playable game. Currently, version 2.5 is the most up to date stable version (released 28 June 2014). Licensed under the GNU General Public License, UFO: Alien Invasion is free software. As of version 2.5, all elements in the downloadable tarballs is released under Free Software licenses. Development is coordinated through SourceForge.net.

Gameplay
Like the original X-COM games, UFO:AI has two main modes of play: Geoscape mode and Tactical (also known as Battlescape) mode.

Geoscape mode deals with base management and strategy. The player directs the activities and finances of PHALANX, controlling bases, installations, aircraft and squads of armed-response troops. Once researched, new technologies may be used in battle against the aliens. Interceptor aircraft can be launched to shoot down UFOs and dispatch dropships in response to alien activity across the globe. The player can build, buy and produce anything available, as long as technology level and budget will allow it. Geoscape mode employs easy-to-use time buttons to control the passage of time, automatically pausing whenever there's an important message. Research will progress and items will be produced as time passes.

In Tactical mode, the player takes command of a team of soldiers in various missions to combat the aliens wherever they might appear. Instead of pushing impersonal armies around on giant maps, the players uses only the team of soldiers assigned to deal with this mission. Tactical mode uses a turn-based system, where the human team and the aliens take turns to make moves. During the player's turn the troops can be ordered to move around, fire their weapons, throw grenades or use other equipment. Each soldier gets a certain number of Time Units (TUs), representing the total time they have to act during the current turn. All actions in Tactical mode require Time Units to perform and once a soldier is out of Time Units, he or she can't do anything more until the next turn. Civilians and other bystanders may be found during the mission, either being targeted by the aliens, or just getting in the way while the troops try to protect them. Saving the game is impossible in tactical mode.