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.
ImageJ - and Other Health Care
Artrage - A simple but lovely natural media painting and sketching program.
Avogadro - Avogadro is a free, fantastically easy to use molecule editor.
PaintTool SAI - Learn to draw in different ways.
ACD/ChemSketch - ACD/ChemSketch for personal and academic use brings the power of our structure drawing package to your home computer.