Check out an astonishing collection of free materials for Blender by Michael Novelo.
The site features tons of procedural and texture-based materials. Each file includes a description of how to use a material. The textures are usually seamless and available with a minimum resolution of 1024x1024px.
The collection has different kinds of materials, like concrete, fabric, fluid, some abstract ones, and many other.
You can find all the materials and learn more about the project here.