Hello,
You may be interested in those projects (in case you don't know them already):
http://ufoai.sourceforge.net/ - open source x-com inspired game, stable version is quite outdated, look at forum for new builds
http://ufopaedia.org/index.php?title=Use.. - extender for original game, fixes bugs and adds new features
http://nefurii.my-place.us/ - reimplementation of game engine, uses graphics from original game. Multiplayer (ufo vs x-com) is implemented. Author wants to implement single player in future.