This is a free, open-source game that simulates a tactical squad with turn-based gameplay. It also supports multiplayer mode.
The UFO2000 project is programmed in portable C, making it known to compile and run without issue on both Linux and FreeBSD. You require data files from the original X-COM to play UFO2000, and the team is currently working on making their own set of graphics, which will be free. If you want to contribute to this effort, then follow the link provided.
In case you don't have access to the original X-COM, the game can still work with data files from X-COM Demo. However, in this case, you can only play on the 'City' map, and the number of units you will be able to use will also be limited. Overall, the game has been well-received by the gaming community, with high ratings given for the graphics, gameplay, and multiplayer support.
Version 0.6.627 / 0.9.1105 Beta: N/A