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 provides many new additions, including map sizes up to 4096 tiles, multi-lingual support, 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 online 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 games. As of version 1.0 it no longer requires any original files from the original Transport Tycoon Deluxe game, replacing the graphics and sounds with community-powered alternatives. It runs on Windows, Mac OS X and Linux, and has been ported to many other platforms including an unofficial port for Android.