About a year ago I found a pretty good fan made game inspired by X-Com: Enemy Unknown called UFO: Alien Invasion. It was still kinda beta and buggy with lot of unimplemented features.
I lost interest after a while and completly forgot about it until this weekend. Turn out they where not idle for this whole time and they made a hell of a game.
http://ufoai.sourceforge.net/It is still not complete but very playable, and the developpers are quite active. The graphics are nothing special, the engine being based on the old quake 2 source, but it is good enough. Many small details have been added that make it even better than X-Com: EU, like persistent equipement for your soldiers, "retry" option in missions, really cool research/weapons descriptions.
The only thing i miss from the old x-com is the destructible environnement... Sadly the quake 2 engine do not make it possible, at least not without some heavy modifications. However the game beign in 3d, there is a fairly accurate modelling of grenade bounce. With some practice you can bounce grenades off a wall to kill/flash bang an alien without exposing yourself.
The AI is decent, and they are working on an improved one for the next update.
It is really worth a try, even in this incomplete state it has totally annihilated my weekend.