An open source remake and expansion of the 1995 Chris Sawyer video game Transport Tycoon Deluxe
It duplicates most of Transport Tycoon Deluxe's features but also has many additions, including a range of map sizes, support for many languages, custom (user-made) AI, downloadable customizations, and a more user friendly interface (for a full list, see this page on the OpenTTD wiki). The game also supports LAN and Internet multiplayer, both co-operative and competitive, for up to 255 players.
It is under ongoing development and is one of the most popular open source game. As of version 1.0 it does not require any graphics and sound files from the original Transport Tycoon Deluxe It runs on Windows, Mac OS X and Linux, and has been ported to other platforms including an unofficial port for Android.