Krita is a professional, free, and open source painting program. It is made by artists that want to see affordable art tools for everyone. It is the full-featured digital painting studio for artists who want to create professional work from start to end. Krita is used by comic book artists, illustrators, concept artists, matte and texture painters, and in the digital VFX industry.
Krita has several features that are unique or a first among free software painting applications: support for colorspaces other than RGB, like CMYK, support for HDR painting, painting assistants, a perspective grid.
Anyone looking to replace Adobe Photoshop with a free and full-featured alternative having a familiar interface and workflow, Krita might be the answer.