GRFMaker is a Windows program that allows creating GRF files for TTDPatch and OpenTTD with a graphical user interface. You will still need to be at home the NFO specification to be able to create GRF:s, but GRFMaker removes some of the hairier bits of NFO coding.
GRFMaker was developed by TT-Forums members gl2 and Szappy as a closed source project, but in spring 2006 development seemed to stop. You can read more in the TT-Forums discussion thread.
In August 2007, the source code for GRFMaker version 3.03B was released by gl2. The originals can be found at Csaboka's TTD pages. A month later, a SVN repository was setup. As of August 2016, the source code can be found at the #openttdcoop DevZone.
These pages are trying to collect the available information about GRFMaker in one place, and to have a place to find updates to the program.