XCOM: Enemy Within
First mentioned by EscapeGoat
While the Enemy Within DLC doesn't change everything about XCOM: Enemy Unknown, the changes it does make are massive in scope. There are new alien units, a new resource called "Meld," and you can even modify your soldier to have superhuman abilities. The DLC also adds a new enemy faction called EXALT. A covert paramilitary organization run by humans, not aliens, EXALT works to undermine XCON at every turn. In addition, the XCOM base can now be assaulted by aliens, with a defeat ending the game. Finally, the DLC adds 47 single-player and eight new multiplayer maps to the original game's offerings.