Take a photo of a dirty grungy surface like an old table, wall, floor and so on.
Open it in photoshop and turn it into a black and white image (image -- adjustments -- desaturate)
Turn up the contast so there is good contrast between black and white.
Define it as a brush (image -- adjustments -- define brush preset) and voila, you have a new grunge brush!
You can also use the spatter filter on images and text to grunge them out.