Vladimír Mrkvica and his team are currently at work on their student film project. Having gained a lot from the CG community in the past they're being really nice and giving away all the assets they create for free.
This week they're giving away a texture pack that contains ten 4K textures of tree bark. All the textures have maps for diffuse, height, roughness, normal and ambient occlusion. They are tiled 24 bit PNG's and can be downloaded here.