Krita is a FREE digital painting and illustration application.
Krita offers CMYK support, HDR painting, perspective grids, dockers, filters, painting assistants, and many other features you would expect.
It is a KDE program for sketching and painting, offering an end–to–end solution for creating digital painting files from scratch by masters. Fields of painting that Krita explicitly supports are concept art, creation of comics and textures for rendering. Modelled on existing real-world painting materials and workflows, Krita supports creative working by getting out of the way and with a snappy response.
It means that Krita as a project is proud to be part of the wonderful KDE community and uses the great framework technology that the KDE community develops.. You can run Krita on Windows, Gnome, XFCE, and if you spend some effort even on OSX.