OpenTTD
An open-source simulation game based on Transport Tycoon Deluxe, letting you build and manage a vast transport empire.
OpenTTD is an open-source reimplementation and expansion of Transport Tycoon Deluxe, the classic business simulation game by Chris Sawyer. Players build and manage transport networks, connecting industries, cities, and stations by rail, road, ship, and air.
The game adds modern features to the original, including large maps, dynamic economy and inflation, an extensive signal and pathfinding system for rail networks, and multiplayer support for up to 255 players on shared networks. NewGRF mods allow players to add custom vehicles, industries, and graphics.
OpenTTD is licensed under GPL-2.0 and is completely free. Unlike some reimplementations, it does not require the original game files: the graphics are included in an open format (with an optional freeware graphic set), so it can be played entirely on its own.