This is by far one of my favorites. It saves time, and effort in looking for grunge brushes.

1. Find a picture. I usually goto Google Images and search for a tree, dirt, sand, grass.. etc. Something with a lot going on in it works best.

2. Copy it to Photoshop, and than goto Image > Adjustments > Threshold. Lower numbers work best. Just mess around with it. If you dont like how it comes out the first time, goto Edit > Step backward than re-do the first part of this step.

3. If you dont want a big brush, than find a small part of the image, and use the rectangular selection tool.

4. Once you have a part of it selected, push ctrl+c on your keyboard, or goto Edit > Copy.

5. File > New. Don't mess around with the heighth and width, because it wont turn out right if you do. Just leave it as it is when the 'new image' window comes up.

6. Paste the selected part of the image that you used Threshold on into the box that comes up. Goto Edit > Define Brush. And, your done.

If you have any questions, just ask me.