Infinite Painter is one of the most feature-packed apps for digital artists, with dozens of brush pre-sets and the ability to create your own, along with layers, blending, editing tools and more, plus the option to export your images as JPEG, PNG, PSD or ZIP.
It has detailed tutorials and guides, although the interface is so simple that you should be able to find your way through most things anyway.
It has superior tools, brushes and interface which help you import and export PSD layers and also add images from the gallery / web.
Many of the advanced features are for the paid version, but it allows you to try them out before you actually buy them.
An interesting tool for the creative digital artist.