I am planning to build a website for tourism in Kenia and thought of a rough concept. I have seen several "rough" websites and I wonder how they make this.
Google 'grunge brushes' and look through for some free ones to import into Photoshop. Better yet, go out with a digital camera and take some pictures of some rough textures - walls, old cars, concrete structures. Desaturate your photos, play with the levels, and then use these templates as ways to apply grunge/roughness to your designs.
If you have a scanner, experiment with scanning materials like oily or crumpled paper, or try textureking.com for some large-format images of textures. Take them into Photoshop, adjust them with the Posterize filter, Sharpen tool, Threshold and Levels to scuff them up. Experiment and you should start seeing the 'grunge' look, or as PettyRider said, you can find grunge brushes anywhere.