Age of Empires II: The Age of Kings came out on September 30th 1999 for the PC and Mac. Soon after its release it became clear that the real-time strategy game was one of the best in the genre and 13 years after its release it still kind of is. Eventually, the title received an official expansion called The Conquerors which added new civilizations, campaigns, units, technologies, maps and modes, among many other features. While that expansion came out in 2000, a new expansion called Forgotten Empires has just been released.
Forgotten Empires adds new campaigns, civilizations (including Italians, Indians, Slavs, Magyars and Incas,) units, maps and technologies. Additional features include a more challenging AI, widescreen support and improved networking. Nevertheless, the best part about Forgotten Empires is that you can download the expansion for free by following this link.