Obsidian Dawn offers a paint-bucket-load of brushes for Adobe Photoshop absolutely free for your downloading pleasure.

Of particular interest to me was the Waterstains brush set. While custom brushes may not be immediately identified as valuable resources to some designers, I think you'll find that the right brush set can really add to your design. They're perfect for creating background images or adding a little flair to you image, as you can see by the image above.

While you're there, be sure to check out the many brush sets available, as well as the Photoshop patterns section.