The three software programs that I use regularly to draw or illustrate are Adobe Illustrator, Photoshop and Corel Painter. When I want to design with a more traditional feel and reach a painterly look my choice is Corel Painter. I have been using it since 2007 when it was available in an actual paint can and known as Painter X. It has evolved since then and their latest is Painter 15 which is an artist's dream!
When you pair it with a graphics tablet you can reproduce the effects of painting and drawing in real time. There are a multitude of brush categories including gouache, watercolor, acrylic, oils, pencil, charcoal and chalk. The interface itself has transformed over time to pretty much match the interface in Photoshop. The ability to work on layers like Illustrator and Photoshop creates a flexible workspace for designing and editing.
For the most part Painter 2015 has focused on improving its best features. In the latest release one of the most obvious improvements is speed. It is 40% faster than its previous version and it is especially noticeable when you use the Real Watercolor brushes. A new feature is the Particle Brush which emulates the look of smoke, fur or just swirls of color. Another new introduction is jitter smoothing which allows finer control over jitter. Adjusting brush strokes is vital and this is an improvement that will allow your brush strokes to look just like you want them to. Finally, the addition of Palette Arrangements is a new way to stay organized in a feature heavy environment. Check out their full list of features here.
Out of the three I think it has the steepest learning curve but it is well worth the effort. As I have mentioned previously, I spent many hours sitting in front of the computer viewing videos from lynda.com when I decided to transition to digital designing. It is a great way to learn at your own pace and become the designer you want to be!
Here is a short video from lynda.com introducing the new Painter 15 interface.