OpenTTD is an open-source remake of the popular Transport Tycoon Deluxe game from Mircoprose.
The game is extended with new features, which include updated in-game mechanics, graphics/interface enhancements and bigger maps. You can read more about them at the Comparison of OpenTTD and TTDPatch features page. The community is very active in updating both the game and its graphics.
You play as the CEO of a transportation company in a simulated world. The game has a strong focus on supply lines. Whether you're trying to supply a factory with the raw resources it needs, or transporting people from one city or another, the world will react and evolve.
OpenTTD can be a relaxing way to construct a digital train set, or it can be a intense competitive experience. Either way it is surprisingly entertaining and worth a download.
The game is available for every major operating system and licensed under the GNU General Public License version 2.0.