Krita - Krita is a professional FREE and open source painting program. It is made by artists that want to seaffordable art tools for everyone. Concept art. texture and matte painters, illustrations and comics.
Blender - Blender is the open source, cross platform suite of tools for 3D creation.
GIMP - GIMP is a multiplatform photo manipulation tool.
ZBrush - ZBrush is a digital sculpting and painting software solution.
Paint.NET - Paint.NET is a free image and photo editing software.
Sculptris - Sculptris: Enter a world of digital art without barriers.