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.
OpenTTD – News

One of the things I love about games like Civilization, SimCity, etc. is that you can create your own worlds fairly quickly and easily, and adapt them to table top games. This is a free version of Transport Tycoon Deluxe. Just design your world, pause the game, and use it for your RPG.