Here is a basic grunge overlay, have fun with it, add it to your papers, color bun, luminosity, color dodge, screen, etc. Double layer it over something else, you will be amazed at the variety you can create with just 1 item.
Here is a visual of the overlay and a link to download:
Here is a visual and link to some samples to view and use:
Besides the above there are a few versions of this page as well.