MyPaint is an easy-to-use painting program which works well with Wacom graphics tablets and other similar devices.

Drawpile is a drawing program that lets you share the canvas with other users in real time.

Gimp Painter is a fork project aiming to add drawing and painting features to GIMP.

MyBrushes is a digital Painting and drawing tool for Mac.

GIMP Paint Studio is a great pack of designing components like brushes, dynamics, gradient etc.