I came across this video tutorial titled Nitty-gritty at Photoshop QuickTips and thought it was fantastic. It uses a technique I've been using for a long time, which is adding noise over an image, but then diffusing it with a Render Clouds filter. And of course, making the image a Smart Object allows you to edit the image with non-destructive freedom.

If you're looking to give your image a gritty, high-contrast look, this tutorial will do the trick.