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OpenTTD main site
OpenTTD is a open source clone of Transport Tycoon Deluxe, integrating loads of new features just like TTDPatch does.
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OpenTTD manual
The manual pages for OpenTTD.
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TT-Forums
The place to talk about Transport Tycoon, 'nuff said.
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TT-MS Headquarters Forum
In case your German is better than your English you might want to look at the German forums.
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GRFCrawler
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.