FireAlpaca is a Digital Painting Software available in 10 languages and compatible with Mac and Windows. Simple tools and controls let you draw an illustration easily.
FotoSketcher is an app for transforming ordinary photos into stunning paintings or drawings. You can add many artistic effects to your images, like oil pastels, watercolors, bilateral filters, oil painting, brushstrokes, etc.
MyPaint is an easy-to-use painting program that works well with Wacom graphics tablets and other similar devices. It has a large brush collection, including charcoal and ink, to emulate real media.
Artweaver is a full-featured painting tool with a huge set of predefined realistic brushes to paint creatively or just experiment.
Krita is an open-source graphics editor for image creation and manipulation designed primarily for digital painting, illustration, concept art, and other creative work.