Programs related to TTDPatch, or otherwise useful for TTD.
I have made a program which can be used to modify TTD's graphics. It will convert the .GRF file into a .PCX file which you can edit with your favourite graphics program. Then you can convert it back and have TTD use your own graphics set. In addition, you can use it to edit signals.grf if you want to modify the look of pre-signals. You can find GRFCodec here
It is still more or less in beta stage and may not work correctly in all cases, though.
Another very useful tool is TTDAlter, written by Owen Rudge. It lets you edit the engines and bridges and other stuff, to make them faster, better, bigger, whatever. It's especially useful in combination with TTDPatch.
You can find it at Owen's Transport Tycoon Station.
When using it with TTDPatch, make sure you check the "Delete TTDLOAD[W].OVL when saving" option, which allow TTDPatch to make use of your changes.
For other tools, please check the Tools page on the TTDPatch site.