From OpenTTD Group:
OpenTTD is an open source simulation game based upon the popular Microprose game "Transport Tycoon Deluxe", written by Chris Sawyer. It attempts to mimic the original game as closely as possible while extending it with new features. As president of a start-up transportation company in 1950, you have a choice of building rail, road, air and maritime transportation routes to build up your transportation empire. Outsmart your competition by beating them to attractive passenger and commodities routes to become the game's highest-ranking transport company by the year 2050. During the course of the game, the player has to build and expand their transport infrastructure. The only infrastructure present on the map at the start of the game are roads within towns, all other infrastructure; ports, stations, airports, rail and depots, have to be built by the player. The tools for building a rail network are particularly powerful and the player has access to many different signal types in order to build a complex and interconnected rail network.
Technological improvements give the player access to newer, faster and more powerful vehicles. In the case of rail transport, new track technology also becomes available over time, first electrified rail, then monorail and maglev track. In general, newer vehicles cost more money to purchase and to run, and the player has to have earned enough money in the earlier stages of the game in order to be able to afford to upgrade their vehicles. The player can optionally start at earlier dates and play on past 2050, although no new technology becomes available. OpenTTD can be played as a single player, against a computer controlled AI, or multiplayer both over a LAN or the Internet. OpenTTD supports multiplayer games for up to 255 players between 15 different transport companies can be played both over a LAN or over the Internet. Each transport company is in competition with each other transport company, and each transport company can be controlled by more than one player at any time. This allows both co-operative and competitive multiplayer games. Competitive team games (e.g. two transport companies, both controlled by three players) are also possible. OpenTTD is licensed under the GNU General Public License version 2.0. For more information, see the file 'COPYING' included with every release and source download of the game.