Kenney - Prototype Textures

As of now, this is the only material pack Kenney has created
I recommend using this with Unity Probuilder's AutoUV feature to keep the textures at a constant size no matter the shape of the objects
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They have many CC0 materials on their page, along with some very nice models
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My personal go-to for beautiful CC0 materials
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Very many free materials, and many work with Adobe Substance 3D to customize the materials further