OpenApoc

OpenApoc is an free, open-source clone of the X-COM: Apocalypse, that requires the original game files to run, licensed under the MIT and written in C++ / SDL2. It was originally founded by PmProg and JonnyH in July 2014, and has since grown in to community.

Key Features:
-Unlimited modding capabilities, which was not possible in the original
-Port the game to any platform you like (windows, linux, android etc)
-Support for modern screen resolutions
-Added a full debug system (hot keys, etc.)
-Added 'more options' menu (which many improvements)
-Added skirmish module (fast fight)
-We can bring, in mods, Julian Gollop cut ideas into the game
-The new engine have ample opportunities for expansion and changes
-High FPS, smooth sound during the game without bugs from original
-No limitations which were in vanilla