Check out this website for a huge archive of Photoshop brushes. Ive used this place for years and it has really changed the look of my work. I tend to focus in on the grunge brushes and the dry paint brushes. The effects can be see in the night Jindu rendering sky that I posted a while ago. They are also great for breaking out of the clean "illustrator" look that is easy to get into when working on portfolios and presentation boards.