I just came across this game while looking for games to play on my Mac. It's pretty cool as its a free game in the style of the old DOS game X-Com. I think it is much better then a lot of the commercial games based on X-Com. Its still a work in progress though mostly complete and works on PC, Mac and Linux. If you liked X-Com and you haven't seen this yet I suggest giving it a try.

http://ufoai.sf.net