http://www.techradar.com/news/download-of-the-day-artweaver-free

Painting software has come a long way in recent years, and there are now superb programs available completely free that offer a realistic simulation of watercolors, oils, pastels, charcoal, and more.

These free tools work well with a mouse, but are even better with a stylus or touchscreen for direct control over your virtual brushes and pens.