Digital painting pioneer John Derry makes & sells excellent Artists’ Brushes & Dry Media brushes for Photoshop, and you already can get some of them for free: go to the Tool Presets panel in Photoshop CS6 and load up “Airbrushes” and “Mixer Brush Pencil.” (There, I just doubled the number of people who know about these hidden gems!)
Somehow brushing features always remain kind of obscure, but in a bid to change that John has just released the new Lynda.com title Photoshop CS6 New Features: Brushes. Here’s a sample in which he covers the brush types (Angle, Fan, etc.) that Photoshop now enables.
There’s also my natural media extension (NKS5) available here, for free:
http://nkurence.com/blog/2012/06/nks5-natural-media-toolkit-for-photoshop-cs5-cs6/
Works in CS5 – 6.