SmoothDraw is a natural painting and digital free-hand drawing software that can produce high quality pictures, with an intuitive UI. It supports many kinds of brushes (pen, pencil, dry media, airbrush, bristle brush, and image hose); retouch tools (burn, dodge, blur, shapen, and smudge). Works with tablets and Tablet PC.
It also supports image transform, adjustments and effects; Layers and Layer blend modes; Free canvas rotation. You can draw and paint in super quality, with special sub-pixel level smoothing for thin lines.