From developer's site:
UFO2000 is free and opensource turn based tactical squad simulation multiplayer game. It is heavily inspired by the famous X-COM: UFO Defense game. While UFO2000 engine was specifically designed to be compatible with the graphics resources and maps from X-COM, you don't need any proprietary data files to play as a new fan-made set of graphics exists and is included in UFO2000 distribution by default (see screenshot), so the game is ready to run out of the box. But if you want an exact X-COM look and feel, you have an option of installing original X-COM and TFTD data files and use them for extending UFO2000 with additional maps, weapon sets and units.
UFO2000 is written in portable C++ and is known to compile and run on Linux, FreeBSD and Windows. Latest versions of UFO2000 can work on various architectures such as 64-bit Linux (AMD64), MacOS X (PowerPC) and Linux powered handhelds (ARM)
Right now only human vs. human multiplayer is supported. This is not so bad, especially considering that you may find human opponents much more challenging than a stupid AI in the original games. Up to this moment UFO2000 development was primarily focused on portability and expandability (the game can be extended using lua scripting language), independence from proprietary data files was also a very important milestone.
More info, news, forums and of course downloads of current beta version:
http://ufo2000.sourceforge.net/
Developer: X-COM fans
Status: opensource
