Simutrans is a free (but not open source) simulation game for Windows, BeOS or Linux that focuses on transport of goods and people. It is written by Hansjörg Malthaner, and it is at version Simutrans 0.83.2.2 (although new releases come out fairly rapidly).
Simutrans features the ability to modify the terrain, build roads and rails, build bus stations and truck depots and it also features many ships, trains and vehicles. You can also play against up to 6 computer players, represented by a color. It is also multilingual and it can be configured to play with day/night cycles and there is even a freeplay option where you can play without fear of bankruptcy.
Like many similar games such as Railroad Tycoon, its only real goal is to create a stable economy and grow as well as avoiding bankruptcy.About the game