So X-COM is discounted on Steam this week. Might as well make a thread about it.

X-COM is a game series about the harrowing experience of fending off an alien invasion. While you have the weight of the world's resources at your back for the moment, the aliens have the advantage of overwhelming firepower and singular devotion to your destruction, are capable of subverting your ground troops and even the Funding Council against you through psychic might, and will happily (in the first game at least; Enemy Unknown is better-designed in this respect) pit your squad of Ricky Redshirts against living tanks twice their size. Expect heavy casualties.

Not enough Link Units!
Wiki about the X-COM series, if you're stuck go here
X-COM on Steam
A set of mods to make X-COM: UFO Defense more playable
OpenXcom, a fan rewrite of UFO Defense to make it more compatible and customizable
Xenonauts, a spiritual successor of X-COM: UFO Defense