In this tutorial, I'm going to show how to create a pretty cool abstract brush.
It isn't very easy to get this right, so you might have to try again a few times.

Part One:
Start with a 500x500 document with a white background.
Go to Filter->Render->Fibers... and use these settings:

Then Filter->Distort->Twirl... and set the angle to about 230 degrees.
You should now have something like this:

Now, select the magic wand tool by pressing "W" set the tolerance to about 33:

Use the wand to select an area similar to this:

Make sure you find a cool enough shape, take your time because this step is critical.
You might have to change the tolerance or maybe even start all over again to get a nice shape.

Once you've selected an area you're happy with:
Select->Feather... and use about 6px.

And while the area is still selected:
Edit->Define Brush Preset...
And name it whatever you want.

And your brush is done!


Part Two: Using the brush
Now that you've created your brush, let's try it out.
Open another document, let's say 500x400 or whatever you feel like acctually.
Add a black background and take out your brush tool (Hotkey: "B")
Select the brush you just created:

And in the brush palette, apply these settings:

And start drawing!

Try to add more layers and draw with different colors and blend them together.
Here's some samples:

If your brush didnt come out very well, try again.
Once you get it somewhat right, you can create pretty awesome effects.