OpenGFX is a set of Base graphics for OpenTTD and is the result of the 8bpp Graphics Replacement Project. OpenGFX provides a full open source set of graphics for the original TTD graphic files. Together with OpenSFX and OpenMSX, OpenGFX allows to play TTD without the need for the original TTD CD.
The latest version of the OpenGFX readme is available at this location: https://github.com/OpenTTD/OpenGFX/blob/master/README.md