Build and Shoot

OpenXcom is an open-source clone of the original UFO: Enemy Unknown (X-Com: UFO Defense in USA), licensed under the GPL and written in C++ / SDL. It was originally founded by SupSuper in February 2010, and has since grown into a small development team surrounded by a very supporting community.

The goal of the project is to bring back the tried and true feel of the original with none of the issues. All the same graphics, sound and gameplay with a brand new codebase written from scratch. This should give it:

-Fixability: Play the game natively without any need for emulators or fancy hacks, with none of the limitations and bugs that plagued the original. No more 80-item-limit, personnel limits, funding overflows, disconnected facilities, broken proximity grenades, floating soldiers, etc.

-Moddability: Tweak the game to your heart’s content. Sure the original was pretty good, but maybe you just think it could be that bit better. A nicer base layout, better laser weapons, maybe challenge yourself with a custom game mode, or just put in all the crazy stuff you’ve always wanted! None of it is hardcoded.

-Flexibility: Port the game to any platform you like, customize it to your liking, or use it to make your own far-fetched remakes. The code is fully documented and open-source so anyone can take a crack at it.

UFO: Enemy Unknown, better known in North America as X-COM: UFO Defense, is a strategic turn-based isometric science fiction videogame, made by Mythos Games and published by MicroProse for MS-DOS back in 1994. It is considered to be one of the greatest strategy games of all time, and is a cult classic. Yes, if you have heard of XCOM: Enemy Unknown, then this one cannot be missed. Although Fraxis had done a good job remaking the series for the newer generations, it missed a lot of features that was crucial and the game itself, according to many XCOM vets, too much streamlined for the modern-day audience.
Basically, the game is set in the near future (back then) of 1999, when aliens started invading Earth, overpowering even the strongest military force. The Japanese tried to counter the threat (this game was Japanese, mind you) with their own spec-ops unit Kiryu Kai, but ultimately failed. A 'Council of Nations' decided to band together and form a elite unit, made up of 'best of the best' and called it...
X-COM.
As the Commander, you will have to manage your base (included but not limited to personnel, facilities, aircrafts, et cetera), fight the alien menace, which usually comes in UFOs of varied sizes and you have to send interceptors, or sometimes terrorizing the human population, et cetera. And to even the odds, you can have scientists to research on more advanced weaponary, armor, gear, psionic abilities, augmentations, and ultimately how to defeat those grey bastards. And of course manufacturing that gear. And to prepare to get your whole team of Colonels slaughtered even before you have got a chance to step out of your Skyranger dropship. The game is very challenging even at the lowest difficulty, and has a high replayability. Overall, amateur or master-strategists-who-live-in-mother's-bunker, will enjoy this game. WARNING, it is oddly addictive.