TexGraph is a free procedural texture generation tool that runs on the Windows platform. TexGraph is described as follows:
TexGraph is a procedural texture creation tool that let you create textures by connecting nodes in a graph. It is designed to be similar to other tools in the market but at the same time be very simple and easy to use and extent by users.
If you are a shader programmer you can create your own nodes or edit the existing ones. Check the github page https://github.com/galloscript/TexGraph-Public and the Programming Custom Nodes section of the user manual for more information.
TexGraph is available for download on Itch.io here. While not open source, TexGraph is very easy to customize. New nodes are simply GLSL scripts, while new models and HDR maps can easily be added to the project.