To those who have said "UFO: Enemy Unknown" / "X-Com: UFO Defense" (i.e. the original X-Com game); I'd like to recommend XPiratez. It's set in the X-Com universe, with plenty of X-Com lore; and it's feature packed with huge amounts of different tech, weapons, aliens, and pretty much everything. The main bad thing about it is that it's just too much content for someone who isn't a fairly serious gamer.