Photoshop has a reputation of being an excellent graphics production tool and photo manipulation tool, but these next tips will focus on how to use Photoshop as a creative tool, too.
Back in art school, I knew people who mixed their own paints, starting by purchasing the raw pigments, then grinding the pigments to a fine powder, then mixing it with the appropriate medium. Messy, potentially toxic, and expensive, but some people want that degree of control over their tools. You can create custom brushes in Photoshop with no fuss, no mess, and it may just spur you into becoming more creative.
1. Creating a custom brush is simple in Photoshop. Open a new file, and make sure the Brush Palette is open on screen. If the Brush Palette is not open, double-click on the brush icon on the toolbar, or select Windows > Palettes > Open Brushes.
Adobe says that you can create a brush up to 999 by 999 by pixels, which would be the digital equivalent of painting with a mop. You'll want to keep your brushes on the smaller side, especially if you'll be using them to create web graphics.
2. For this example, I opened a small file, 100 pixels by 100 pixels, and selected Filter > Add Noise, using the Monochrome setting. I repeated the Noise filter (ctrl+f) several times.
3. Through trial and error, I found that brushes with soft edges were much more useful than brushes with hard edges. To soften the edges of the brush, select an area of the image (about a quarter of the way in from all of the edges), then select Select > Feather, then Select > Inverse. Fill the selected area with white.
4. Select the area of the image that you want to use as a brush. Next, from the flyout menu from the Brush Palette, select Define Brush. A new icon will appear on the brush palette for your new brush. You can save your new set of brushes by selecting Save Brushes from the palette.
Some general notes about creating brushes: I've found that it helps to add to the default brush palette, rather than create an entirely new brush palette. I use the regular default brushes so much that it's just easier to add to the existing brushes, rather than having to load a new set. Also, it helps to test out the brush to see if it does what you think it should before you save it. You can always delete a brush from the palette by selecting the icon for the brush, then selecting Delete Brush from the flyout menu on the Brush Palette.