I'll link to a few custom brush sets on Deviant Art that I enjoy at the end of the entry, but first lets walk through the brushes panel:
This panel has a slew of options that you can play with when you are painting. While this isn't as strait forward as a program like painter, you can still achieve great effects. Mixing these up can turn a very simple shape thats defined as a brush into a painterly and layered stroke.
Here's a visualization of these options:
As you can see, these options can have drastic effects on a very simple brush.
Those basic options are just the start of your brush making adventures. Mess with the sliders and and options on your brushes to get new effects when you are bored. If you find something you like, be sure to click the little locks next to the presets so PS remembers your adjustments.
The brush that may stick out from that pack is the textured brush. Defining patterns in photoshop is great for experimenting, you can fill canvases for a quick more interesting base to work on, or you can apply them to brushes like the one above.
To make a textures brush, the first thing you need are patterns. Heres an old image I made for making seamless tiles:
While the tiles here are small, you can use this method on a square of any size. Just make sure you properly divide the canvas in half horizontally and vertically for the offset. It will probably be worth your while to make patterns as high res as you can, for use in print-ready work.
After you make a seamless tile go to Edit>Define Pattern, once it is saved you will be able to apply it to your brush. For me, these are the most important settings for the texture of the brush:
If you have a texture that you want to be clear and never muddied into a solid, be sure that "Texture Each Tip" is off.
One last little chart, This one is for Color Dynamics. Its a difficult setting to control and use, but it does some interesting things, its nice if you feel like your strokes could use a speckle of color.
Fade is a nice way to add a gradient to a stroke of the brush, so its a great thing to know about and play with.
I hope these little charts are helpful reference. As promised here are some of my favorite brushsets:
If there are some great brushes you've stumbled across on the internet, Please comment!