UFO: Alien Invasion
Since this is a game that has rather absorbed my attention for the last few days, it seems only fair to give it a mention here.
Older games players may remember the X-Com Series (starting with "UFO: Enemy Unknown" here in the UK) in which aliens are surrepticiously visiting Earth, mutilating people and abducting cattle - or was it the other way around? X-Com is set up as a united defence against the alien threat and your role is to manage your bases, research and production in one half of the game and to lead your teams of warriors into battle in the other half.
UFO: Alien Invasion takes this old game and reinvents it. This is no straight port with flashier graphics but a new game altogether. The aliens are different. The combat is more clever. The weapons are vastly more numerous. The research tree makes a modicum of sense! It's a rather different beast to the original and I love it.
Opensource, this is a game still constantly in development. X-Com fans, you'll love this. Developers, you're welcome to pitch in and help with the latest version. Visit them at http://ufoai.sourceforge.net/ and see why I've been out of contact with the human race for the last few days.