OpenTTD is a clone of the Microprose game "Transport Tycoon Deluxe", a popular game originally written by Chris Sawyer. It attempts to mimic the original game as closely as possible while extending it with new features.
OpenXcom is a new engine for the old game UFO: Enemy Unknown (X-Com: UFO Defense in the USA),
a popular strategy videogame made by Mythos/Microprose in which you take control of the X-Com
organization to fight off the alien menace invading Earth.