I am always blown away with digital painting-- how realistic they can be! Mimicking the way paint smudges or how a crayon is a little rugged. I've now just discovered the Nomad Brush, which is a digital brush that works on iPads and iPhones (and possibly other tablets). Check out the video below to get a sense of what I'm talking about!
Nomad Brush: A Portrait from Don Lee on Vimeo.