OpenXCom has taken its time. When we last posted about it—for its 0.9 release —it had been in the works for four years. That was over a year ago, meaning that this new v1.0 release has taken... well, you should be able to do the maths. It's an open-source clone of the original XCOM: UFO Defense / UFO: Enemy Unknown, and lets you play Microprose's original, brutal tactical strategy in an easier-to-use engine.
Importantly, OpenXcom is the same game, with the same difficulty in place. But, in addition to that, it offers a slightly more customisable experience, provides a series of bug-fixes and, most significantly, a nice new wrapper that isn't designed for twenty-year-old operating systems.
As its so unapologetically the exact same game, you will need a vanilla version of the original. You can grab a digital copy from Steam , or, alternatively, you can order a copy from the secret global black market for just two Sectoid corpses.