GIMP has a very powerfull brush tool. It is already far far
better than most of the expensive rivals. 

Brush file formats native to the GIMP are GIH, GPB and GBR. 
Brush files must be placed in your ~/.gimp/brushes directory 
to be available for your use in the Brush Selection dialog. 

Use Refresh in Brush Selection Dialog to see the new brush(es) 
without restarting GIMP.

Gimp Sheep Brush includes 16 images arranged into angular and 
random arrays.

You can freely distribute these brushes as long as you retain
this disclaimer. 

Happy Gimping...
Nevit Dilmen
http://start.at/nevit