ambientCG
ambientcg.com
A large library of free CC0 PBR textures, HDRI lighting maps and 3D models for use in games, animation and visualisation.
Why it is useful
Everything is released into the public domain under CC0, so you can use it in commercial projects without attribution. The materials come in the standard maps that Blender, Unreal, Unity and most other 3D tools expect.