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.
Aseprite - Aseprite is an art program dedicated to the creation of pixel art.
MyPaint - MyPaint is a fast, distraction-free, and easy painting tool for digital artists.
Pixelorama - Free and open source sprite editor and animator, ideal for pixel art.
GIMP - GIMP is a multiplatform photo manipulation tool.
Piskel - Piskel is a website where designers online create sprites or pixel art.