Material Maker is a procedural PBR (Physically Based Rendering) materials authoring tool, based on the Godot Engine.
Materials are defined as graphs where nodes create or transform textures, and can be exported for the Godot, Unity and Unreal game engines.
There are more than 200 available nodes, and new nodes can be created either by connecting and grouping existing nodes, or by describing them in GLSL.
Starting from release 0.94, Material Maker can also be used to paint PBR materials on 3D models. Brushes are also described as graphs using the same nodes as PBR materials plus a few painting specific filters and transforms that depend on the mesh geometry.
This website can be used to browse materials and brushes created by the community and open them in Material Maker, and share your own (detailed instructions here).
If you haven't used Material Maker yet, you may want to watch this tutorial:
Here are a few materials created by Pavel Oliva with Material Maker 0.93: