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.
Sketch - Professional digital design for Mac.
Artrage - A simple but lovely natural media painting and sketching program.
Inkscape - Inkscape is a free, open source professional vector graphics editor for Windows, Mac OS X and Linux.
Procreate - Procreate is the most powerful and intuitive digital illustration App available for iPad®.
Adobe Photoshop - Adobe Photoshop is a webtop application for editing images and photos online.