OpenTTD is a free/open source video game based on TTDPatch, which was a humongous Game Mod for Transport Tycoon Deluxe. In fact, the two shared the majority of their code, and were maintained in parallel by the same dev team for several years, so both are covered by this article, but since OpenTTD is still in development, and can be used by anyone, not just those who have an old copy of the original game, it will be the primary focus.
TTDPatch started as an attempt to collect all the best patches for Transport Tycoon Deluxe, and it gradually grew to be much larger than the original game. At this point, some folks decided that it might be worthwhile to replace the last few bits of the original that hadn't already been bypassed, and make a complete standalone game that anyone could play. A few years later, in 2004, an initial version of OpenTTD was released, which could run on modern operating systems. By 2010, all the original graphics and music had been replaced, and OpenTTD was completely standalone.
Like other games in the Transport Tycoon family, OpenTTD is a business management Simulation Game. The player runs a transport company which builds trains, buses, trucks, ships, and airplanes to deliver passengers and goods to various locations on a map in order make money. The player may compete with companies run by other players, or with one or more of the available AI players.
While OpenTTD may have started life as a patch to Transport Tycoon Deluxe, it has been so heavily modified and enhanced that calling it a "derivative" is a bit misleading. Saying that it is "inspired by" is probably more accurate. OpenTTD has revamped almost every element of the original game [such as...]