OpenTTD is a open source clone of Transport Tycoon Deluxe, integrating loads of new features just like TTDPatch does.

The manual pages for OpenTTD.

This is the location where to download JGR's patch pack from. This is a special version of OpenTTD with a number of extensions and improvements, among them additional signal types, larger maps, more complex order options and a lot more.

The Wiki for JGR's patch pack documents the additional features and should be worked through before playing with the patch pack.

The place to talk about Transport Tycoon, 'nuff said.

In case your German is better than your English you might want to look at the German forums.

Any new graphics for TTDX are stored in grf-format and can be loaded with the help of TTDPatch. When you are looking for new buildings, vehicles or other stuff, take a look there.