Verve - Digital Painting Software 
Verve is a new, free painting program, for Windows OS, created by Taron.
This painting program is unique, to me, for several reasons...
- Allows you to create believable "traditional" look, much easier than other software I've used.
- Good control and variety over the "fluid dynamics", to make paint dryer, thinner, more opaque, etc.
- Many creative and experimental options, such as: 1) The ability to spin the canvas, to create spiral or circular patterns. 2) Mirroring, tiling, and several interesting viewing options. 3) Brush presets which are simple but can be used for a variety of applications and techniques - you can adjust individually bristle count, weight, etc. 4) Paint reacts to canvas texture.
There are many good things to say about this software. You can learn more about Verve on Taron's official forum: http://www.taron.de/forum/Jun 16th 2014 