Photoshop resource: Brushes (free)
I actually posted this in another thread in here, but I feel like it will be more easily accessable in it's own thread.
I get most from deviantart.com. There are some amazing talents on there. I also have a few other sites I go to, or if I'm looking for something in particular, I google "_____ photoshop brush"
http://sideduck.blogspot.com/ This is a great resource for free brushes.
You can find everything on deviant art!