MyPaint is a fast and easy open-source graphics application for digital painters. It lets you focus on the art instead of the program. You work on your canvas with minimum distractions, bringing up the interface only when you need it.
MyPaint's brush engine is fast and almost infinitely configurable to simulate many natural media brushes or experimental ‘unrealistic’ ones. It ships with a variety of pre-made brushes by prominent artists so you don't have to configure your own if you don't want to.